Semiconductor device and method for manufacturing semiconductor device

ABSTRACT

A highly reliable semiconductor device is manufactured by giving stable electric characteristics to a transistor in which an oxide semiconductor film is used for a channel. An oxide semiconductor film which can have a first crystal structure by heat treatment and an oxide semiconductor film which can have a second crystal structure by heat treatment are formed so as to be stacked, and then heat treatment is performed; accordingly, crystal growth occurs with the use of an oxide semiconductor film having the second crystal structure as a seed, so that an oxide semiconductor film having the first crystal structure is formed. An oxide semiconductor film formed in this manner is used for an active layer of the transistor.

TECHNICAL FIELD

The present invention relates to a semiconductor device which includes acircuit including a semiconductor element such as a transistor, and amethod for manufacturing the semiconductor device. For example, thepresent invention relates to a power device which is mounted on a powersupply circuit; a semiconductor integrated circuit including a memory, athyristor, a converter, an image sensor, or the like; and an electronicdevice on which an electro-optical device typified by a liquid crystaldisplay panel, a light-emitting display device including alight-emitting element, or the like is mounted as a component.

In this specification, a semiconductor device means all types of deviceswhich can function by utilizing semiconductor characteristics, and anelectro-optical device, a light-emitting display device, a semiconductorcircuit, and an electronic device are all semiconductor devices.

BACKGROUND ART

A transistor formed over a glass substrate or the like is manufacturedusing amorphous silicon, polycrystalline silicon, or the like, astypically seen in a liquid crystal display device. Although a transistorincluding amorphous silicon has low field effect mobility, it can beformed over a larger glass substrate. On the other hand, although atransistor including polycrystalline silicon has high field effectmobility, it is not suitable for being formed over a larger glasssubstrate.

In contrast to a transistor including silicon, attention has been drawnto a technique by which a transistor is manufactured using an oxidesemiconductor and is applied to an electronic device or an opticaldevice. For example, Patent Document 1 and Patent Document 2 disclose atechnique by which a transistor is manufactured using zinc oxide or anIn—Ga—Zn—O-based oxide as an oxide semiconductor and is used as aswitching element of a pixel or the like of a display device.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-96055

DISCLOSURE OF INVENTION

Electric characteristics of a transistor are easily affected by thecondition of an interface between an oxide semiconductor film serving asan active layer and a gate insulating film in contact with the oxidesemiconductor film. During or after manufacture of the transistor, ifthe interface where the gate insulating film is in contact with theoxide semiconductor film, that is, the gate-electrode-side interface ofthe oxide semiconductor film is in an amorphous state, the structuralcondition is easily changed by an influence of temperature or the likein the manufacturing process and electric characteristics of thetransistor are likely to be unstable.

Further, electric characteristics of a transistor in which an oxidesemiconductor film is used for a channel can be changed by irradiationwith visible light or ultraviolet light.

In view of such problems, an object of one embodiment of the presentinvention is to provide a semiconductor device including a transistor inwhich the condition of an interface between an oxide semiconductor filmand a gate insulating film in contact with the oxide semiconductor filmis favorable, and to provide a method for manufacturing thesemiconductor device. Further, an object of one embodiment of thepresent invention is to manufacture a highly reliable semiconductordevice by giving stable electric characteristics to a transistor inwhich an oxide semiconductor film is used for a channel. Further, anobject of one embodiment of the present invention is to provide amanufacturing process of a semiconductor device, which enables massproduction of highly reliable semiconductor devices with the use of alarge-sized substrate such as a mother glass.

In one embodiment of the present invention, in order to make thecondition of an interface between an oxide semiconductor film and aninsulating film (a gate insulating film) in contact with the oxidesemiconductor film favorable, a region with high crystallinity is formedat least in the vicinity of the interface of the oxide semiconductorfilm. Accordingly, a highly reliable semiconductor device having stableelectric characteristics can be manufactured.

Further, as a method for improving the crystallinity of the oxidesemiconductor film, an oxide semiconductor film having a second crystalstructure may be provided in part of the oxide semiconductor film. Thesecond crystal structure is a wurtzite crystal structure. An oxidesemiconductor film which can have the second crystal structure is easilycrystallized by heat treatment and has high crystallinity as compared toan oxide semiconductor film which can have a first crystal structure,the first crystal structure being selected from a non-wurtzitestructure, a YbFe₂O₄ structure, a Yb₂Fe₃O₇ structure, and deformedstructures of the foregoing structures.

The oxide semiconductor film which can have the first crystal structureby heat treatment and the oxide semiconductor film which can have thesecond crystal structure by heat treatment are formed so as to bestacked, and then heat treatment is performed; thus, crystal growthoccurs in the oxide semiconductor film which can have the first crystalstructure by heat treatment with the use of the oxide semiconductor filmhaving the second crystal structure as a seed, so that an oxidesemiconductor film having the first crystal structure is formed.

The heat treatment is performed at a temperature higher than or equal to150° C. and lower than or equal to 650° C., preferably higher than orequal to 200° C. and lower than or equal to 500° C.

Instead of performing the heat treatment for crystallization, the oxidesemiconductor film can be formed by a sputtering method while beingheated.

In this manner, for example, a layer including at least a second oxidesemiconductor film is provided in an oxide semiconductor stack in whichoxide semiconductor films are stacked and heat treatment is performed onthe oxide semiconductor stack, whereby an oxide semiconductor film withhigh crystallinity can be obtained.

In addition, the thickness of the second oxide semiconductor film isgreater than or equal to a thickness of one atomic layer and less thanor equal to 10 nm, preferably greater than or equal to 2 nm and lessthan or equal to 5 nm.

In the above structure, the oxide semiconductor film isnon-single-crystal, is not entirely in an amorphous state, and includesat least crystal having c-axis alignment.

One embodiment of the present invention is a method for manufacturing asemiconductor device including a transistor. In the method, a firstoxide semiconductor film is formed over an insulating surface, and thena second oxide semiconductor film is formed; after that, first heattreatment is performed, so that an oxide semiconductor film having afirst crystal structure and an oxide semiconductor film having a secondcrystal structure are formed. Next, a third oxide semiconductor film isformed over the oxide semiconductor film having the second crystalstructure, and then second heat treatment is performed, so that an oxidesemiconductor film having a third crystal structure is formed. The stackof the oxide semiconductor film having the first crystal structure, theoxide semiconductor film having the second crystal structure, and theoxide semiconductor film having the third crystal structure is used as achannel region of the transistor.

The crystal structures of the oxide semiconductor film having the firstcrystal structure and the oxide semiconductor film having the thirdcrystal structure are each any one of a YbFe₂O₄ structure, a Yb₂Fe₃O₇structure, and a non-wurtzite structure. The crystal structure of theoxide semiconductor film having the second crystal structure is awurtzite structure.

The temperatures of the first heat treatment and the second heattreatment are each higher than or equal to 150° C. and lower than orequal to 650° C., preferably higher than or equal to 200° C. and lowerthan or equal to 500° C. Therefore, a mother glass which is alarge-sized substrate can be used as a substrate.

Each of the oxide semiconductor film having the first crystal structure,the oxide semiconductor film having the second crystal structure, andthe oxide semiconductor film having the third crystal structure isnon-single-crystal, is not entirely in an amorphous state, and includesa c-axis-aligned crystal region. That is, each of the oxidesemiconductor films has an amorphous region and a c-axis-aligned crystalregion.

The oxide semiconductor film having the second crystal structure, whichhas a wurtzite crystal structure, is easily crystallized by heattreatment and has high crystallinity as compared to the oxidesemiconductor film having the first crystal structure and the oxidesemiconductor film having the third crystal structure. Further, theoxide semiconductor film having the second crystal structure includesbonds that form a hexagonal shape in a plane in the a-b plane. Inaddition, layers including hexagonal bonds are stacked and bonded in thethickness direction (the c-axis direction), so that c-axis alignment isobtained. Therefore, when crystal growth is caused in the first oxidesemiconductor film and the third oxide semiconductor film by heatingwith the use of the oxide semiconductor film having the second crystalstructure that is a wurtzite crystal structure as a seed, the oxidesemiconductor film having the first crystal structure and the oxidesemiconductor film having the third crystal structure can be formed sothat the crystal axes thereof are generally aligned with the crystalaxis of the oxide semiconductor film having the second crystal structurethat is a wurtzite crystal structure. The oxide semiconductor filmhaving the first crystal structure and the oxide semiconductor filmhaving the third crystal structure each include bonds that form ahexagonal shape in a plane in the a-b plane as in the case of the oxidesemiconductor film having the second crystal structure. In addition,layers including hexagonal bonds are stacked and bonded in the thicknessdirection (the c-axis direction), so that c-axis alignment is obtained.

By forming a gate insulating film over the above oxide semiconductorstack and forming a gate electrode over the gate insulating film, atransistor can be manufactured. As a result, the oxide semiconductorstack has high crystallinity and evenness at the interface with the gateinsulating film and thus has stable electric characteristics;accordingly, a highly reliable transistor can be obtained.

By forming a gate insulating film over a gate electrode and forming theabove oxide semiconductor stack over the gate insulating film, atransistor can be manufactured. As a result, the oxide semiconductorstack has high crystallinity and evenness at the interface with the gateinsulating film and thus has stable electric characteristics;accordingly, a highly reliable transistor can be obtained.

The stack of the oxide semiconductor films each of which includes ac-axis-aligned crystal region having hexagonal bonds in the a-b plane isused for a channel region of a transistor, whereby a transistor in whichthe amount of change in the threshold voltage between before and afterlight irradiation or a bias-temperature stress (BT) test performed onthe transistor is small and which has stable electric characteristicscan be manufactured.

According to one embodiment of the present invention, a semiconductordevice including a transistor in which the condition of an interfacebetween an oxide semiconductor film and a gate insulating film incontact with the oxide semiconductor film is favorable can bemanufactured. Further, a semiconductor device having stable electriccharacteristics can be manufactured. Further, mass production of highlyreliable semiconductor devices can be realized with the use of alarge-sized substrate such as a mother glass.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a top view and a cross-sectional view, respectively,illustrating a semiconductor device which is one embodiment of thepresent invention;

FIGS. 2A to 2C are cross-sectional views illustrating a method formanufacturing a semiconductor device which is one embodiment of thepresent invention;

FIGS. 3A and 3B each show a crystal structure according to oneembodiment of the present invention;

FIGS. 4A to 4C each show a crystal structure according to one embodimentof the present invention;

FIGS. 5A and 5B are each a HAADF-STEM image showing a crystal structureaccording to one embodiment;

FIGS. 6A and 6B are each a HAADF-STEM image showing a crystal structureaccording to one embodiment;

FIGS. 7A and 7B are a top view and a cross-sectional view, respectively,illustrating a semiconductor device which is one embodiment of thepresent invention;

FIGS. 8A to 8C are cross-sectional views illustrating a method formanufacturing a semiconductor device which is one embodiment of thepresent invention;

FIGS. 9A and 9B are a top view and a cross-sectional view, respectively,illustrating a semiconductor device which is one embodiment of thepresent invention;

FIGS. 10A to 10E are cross-sectional views illustrating a method formanufacturing a semiconductor device which is one embodiment of thepresent invention;

FIGS. 11A and 11B are a top view and a cross-sectional view,respectively, illustrating a semiconductor device which is oneembodiment of the present invention;

FIGS. 12A to 12D are cross-sectional views illustrating a method formanufacturing a semiconductor device which is one embodiment of thepresent invention;

FIGS. 13A and 13B are a top view and a cross-sectional view,respectively, illustrating a semiconductor device which is oneembodiment of the present invention;

FIGS. 14A to 14D are cross-sectional views illustrating a method formanufacturing a semiconductor device which is one embodiment of thepresent invention;

FIGS. 15A and 15B are a top view and a cross-sectional view,respectively, illustrating a semiconductor device which is oneembodiment of the present invention;

FIGS. 16A to 16D are cross-sectional views illustrating a method formanufacturing a semiconductor device which is one embodiment of thepresent invention;

FIG. 17 is a cross-sectional view illustrating a semiconductor devicewhich is one embodiment of the present invention;

FIGS. 18A to 18C are a block diagram and circuit diagrams illustratingone embodiment of the present invention;

FIGS. 19A to 19C are each a cross-sectional view illustrating oneembodiment of the present invention; and

FIGS. 20A to 20D each illustrate one embodiment of an electronic device.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in detail withreference to the accompanying drawings. Note that the present inventionis not limited to the following description and it will be easilyunderstood by those skilled in the art that the modes and details of thepresent invention can be modified in various ways without departing fromthe spirit and scope thereof. Therefore, the present invention shouldnot be construed as being limited to the description in the followingembodiments. Note that in structures of the present invention describedhereinafter, the same portions or portions having similar functions aredenoted by the same reference numerals in different drawings, anddescription thereof is not repeated.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales.

Note that terms such as “first”, “second”, and “third” in thisspecification are used in order to avoid confusion among components, andthe terms do not limit the components numerically. Therefore, forexample, the term “first” can be replaced with the term “second” or“third” as appropriate.

Embodiment 1

In this embodiment, a transistor in which an oxide semiconductor filmover an insulating surface is used for a channel and a manufacturingmethod thereof will be described with reference to FIGS. 1A and 1B andFIGS. 2A to 2C. FIG. 1B is a cross-sectional view illustrating astructure of a transistor which is one embodiment of a structure of asemiconductor device, and corresponds to a cross-sectional view alongdashed-dotted line A-B in FIG. 1A which is a top view. Note that in FIG.1A, a substrate 101, an oxide insulating film 102, a gate insulatingfilm 107, and an insulating film 109 are not illustrated. FIGS. 2A to 2Care cross-sectional views illustrating a manufacturing process of thetransistor illustrated in FIG. 1B.

The transistor illustrated in FIG. 1B includes the oxide insulating film102 formed over the substrate 101; an oxide semiconductor stack 105formed over the oxide insulating film 102; a pair of electrodes 106which is formed over the oxide semiconductor stack 105 and functions asa source electrode and a drain electrode; the gate insulating film 107formed over the oxide insulating film 102, the oxide semiconductor stack105, and the pair of electrodes 106; and a gate electrode 108 whichoverlaps with the oxide semiconductor stack 105 with the gate insulatingfilm 107 positioned therebetween. Further, the insulating film 109 whichcovers the gate insulating film 107 and the gate electrode 108 may beprovided.

The oxide semiconductor stack 105 is characterized in that an oxidesemiconductor film 105 a having a first crystal structure, which is incontact with the oxide insulating film 102, and an oxide semiconductorfilm 105 b having a second crystal structure, which is in contact withthe oxide semiconductor film 105 a having the first crystal structure,are stacked.

Further, the oxide semiconductor stack 105 is characterized in thatcrystal growth has occurred in the oxide semiconductor film 105 a havingthe first crystal structure with the use of the oxide semiconductor film105 b having the second crystal structure as seed crystal.

The oxide semiconductor film 105 b having the second crystal structureincludes trigonal and/or hexagonal crystals.

In other words, both the oxide semiconductor film having the secondcrystal structure and the oxide semiconductor film having the firstcrystal structure include trigonal and/or hexagonal crystal; therefore,a hexagonal lattice image can be observed from the c-axis direction.

Note that each of the oxide semiconductor film 105 a having the firstcrystal structure and the oxide semiconductor film 105 b having thesecond crystal structure is non-single-crystal, is not entirely in anamorphous state, and includes a c-axis-aligned crystal region.

Next, a method for manufacturing the transistor in FIG. 1B will bedescribed with reference to FIGS. 2A to 2C.

As illustrated in FIG. 2A, after the oxide insulating film 102 is formedover the substrate 101, a first oxide semiconductor film 103 a is formedover the oxide insulating film 102, and a second oxide semiconductorfilm 103 b is formed over the first oxide semiconductor film 103 a.

It is necessary that the substrate 101 have at least heat resistancehigh enough to withstand heat treatment to be performed later. In thecase where a glass substrate is used as the substrate 101, a substratewith a strain point higher than or equal to 730° C. is preferably used.As a material for the glass substrate, a glass material such asaluminosilicate glass, aluminoborosilicate glass, or barium borosilicateglass is used, for example. Note that a glass substrate containing BaOand B₂O₃ so that the amount of BaO is larger than that of B₂O₃ ispreferably used. For mass production, a mother glass of the eighthgeneration (2160 mm×2460 mm), the ninth generation (2400 mm×2800 mm or2450 mm×3050 mm), the tenth generation (2950 mm×3400 mm), or the like ispreferably used as the substrate 101. The mother glass drasticallyshrinks when the treatment temperature is high and the treatment time islong. Thus, in the case where mass production is performed with the useof the mother glass, the preferable heating temperature in themanufacturing process is lower than or equal to 600° C., furtherpreferably lower than or equal to 450° C.

Instead of the glass substrate, a substrate formed of an insulator, suchas a ceramic substrate, a quartz substrate, or a sapphire substrate canbe used. Alternatively, crystallized glass or the like can be used.Further alternatively, a substrate obtained by forming an insulatingfilm over a surface of a semiconductor substrate such as a silicon waferor a surface of a conductive substrate formed of a metal material can beused.

Note that in the case where a glass substrate including an impurity suchas an alkali metal is used as the substrate 101, a nitride insulatingfilm such as a silicon nitride film or an aluminum nitride film may beformed between the substrate 101 and the oxide insulating film 102 inorder to prevent entry of an alkali metal. The nitride insulating filmcan be formed by a CVD method, a sputtering method, or the like. Sincean alkali metal such as lithium, sodium, or potassium is an impurity foran oxide semiconductor film to be formed later, the content of such analkali metal is preferably small.

The oxide insulating film 102 is formed using an oxide insulating filmfrom which part of contained oxygen is released by heating. The oxideinsulating film from which part of contained oxygen is released byheating is preferably an oxide insulating film which contains oxygen atan amount exceeding the amount of oxygen in its stoichiometriccomposition. With the oxide insulating film from which part of containedoxygen is released by heating, oxygen can be diffused to the first oxidesemiconductor film 103 a and the second oxide semiconductor film 103 bby heating. Typical examples of the oxide insulating film 102 includefilms of silicon oxide, silicon oxynitride, silicon nitride oxide,aluminum oxide, aluminum oxynitride, gallium oxide, hafnium oxide, andyttrium oxide.

From the oxide insulating film which contains oxygen at an amountexceeding the amount of oxygen in its stoichiometric composition, partof the oxygen is released by heating. The amount of oxygen released atthis time which is converted into oxygen atoms is greater than or equalto 1.0×10¹⁸ atoms/cm³, preferably greater than or equal to 1.0×10²⁰atoms/cm³, further preferably greater than or equal to 3.0×10²⁰atoms/cm³ in thermal desorption spectroscopy (TDS) analysis.

Here, a method by which the amount of released oxygen in the case ofbeing converted into oxygen atoms is measured using TDS analysis will bedescribed.

The amount of released gas in TDS analysis is proportional to theintegral value of a spectrum. Therefore, the amount of released gas canbe calculated from the ratio of the integral value of a spectrum of anoxide insulating film to the reference value of a standard sample. Thereference value of a standard sample refers to the ratio of the densityof a predetermined atom contained in a sample to the integral value of aspectrum.

For example, the number of released oxygen molecules (N(O₂)) from anoxide insulating film can be found according to Numerical Expression 1with the TDS analysis results of a silicon wafer containing hydrogen ata predetermined density which is the standard sample and the TDSanalysis results of the oxide insulating film. Here, all spectra havinga mass number of 32 which are obtained by the TDS analysis are assumedto originate from an oxygen molecule. CH₃OH, which is given as a gashaving a mass number of 32, is not taken into consideration on theassumption that it is unlikely to be present. Further, an oxygenmolecule including an oxygen atom having a mass number of 17 or 18 whichis an isotope of an oxygen atom is not taken into consideration eitherbecause the proportion of such a molecule in the natural world isminimal.

N(O₂)=N(H₂)/S(H₂)×S(O₂)×α  (Numerical Expression 1)

N(H₂) is the value obtained by conversion of the number of hydrogenmolecules released from the standard sample into density. S(H₂) is anintegral value of a spectrum when the standard sample is analyzed byTDS. Here, the reference value of the standard sample is set toN(H₂)/S(H₂). S(O₂) is an integral value of a spectrum when the oxideinsulating film is analyzed by TDS. α is a coefficient which influencesthe intensity of the spectrum in the TDS analysis. Refer to JapanesePublished Patent Application No. H6-275697 for details of NumericalExpression 1. Note that the amount of released oxygen from the oxideinsulating film is measured with a thermal desorption spectroscopyapparatus produced by ESCO Ltd., EMD-WA1000S/W with the use of a siliconwafer containing hydrogen atoms at 1×10¹⁶ atoms/cm³ as the standardsample.

Further, in the TDS analysis, part of oxygen is detected as an oxygenatom. The ratio between oxygen molecules and oxygen atoms can becalculated from the ionization rate of the oxygen molecules. Note that,since the above α includes the ionization rate of the oxygen molecules,the number of the released oxygen atoms can also be estimated throughthe evaluation of the number of the released oxygen molecules.

Note that N(O₂) is the number of the released oxygen molecules. For theoxide insulating film, the amount of released oxygen in the case ofbeing converted into oxygen atoms is twice the number of the releasedoxygen molecules.

The thickness of the oxide insulating film 102 is greater than or equalto 50 nm, preferably greater than or equal to 200 nm and less than orequal to 500 nm. With the use of the thick oxide insulating film 102,the amount of oxygen released from the oxide insulating film 102 can beincreased, and defects at the interface between the oxide insulatingfilm 102 and an oxide semiconductor film to be formed later can bereduced.

The oxide insulating film 102 is formed by a sputtering method, a CVDmethod, or the like. The oxide insulating film from which part ofcontained oxygen is released by heating is easily formed by a sputteringmethod, which is preferable.

When the oxide insulating film from which part of contained oxygen isreleased by heating is formed by a sputtering method, the amount ofoxygen in a deposition gas is preferably large, and oxygen, a mixed gasof oxygen and a rare gas, or the like can be used. Typically, the oxygenconcentration in the deposition gas is preferably higher than or equalto 6% and lower than or equal to 100%.

The first oxide semiconductor film 103 a is formed using an oxidesemiconductor film which can include trigonal and/or hexagonal crystaland have the first crystal structure by heating.

As the first oxide semiconductor film 103 a, a four-component metaloxide such as an In—Sn—Ga—Zn—O film; a three-component metal oxide suchas an In—Ga—Zn—O film, an In—Sn—Zn—O film, an In—Al—Zn—O film, aSn—Ga—Zn—O film, an Al—Ga—Zn—O film, or a Sn—Al—Zn—O film; atwo-component metal oxide such as an In—Zn—O film, a Sn—Zn—O film, anAl—Zn—O film, or an In—Ga—O film; or the like can be used. Further, SiO₂may be contained in the above oxide semiconductor. In thisspecification, for example, an In—Ga—Zn—O film means an oxide filmcontaining indium (In), gallium (Ga), and zinc (Zn).

The first oxide semiconductor film 103 a is formed using an oxidesemiconductor film which can include trigonal and/or hexagonal crystaland have any one crystal structure of a non-wurtzite structure, aYbFe₂O₄ structure, a Yb₂Fe₃O₇ structure, and deformed structures of theforegoing structures by heating.

As an example of the oxide semiconductor film having the first crystalstructure, an In—Ga—Zn—O film that is a three-component metal oxideincludes trigonal and/or hexagonal non-wurtzite crystal. In addition,examples of the In—Ga—Zn—O film that is a three-component metal oxideinclude InGaZnO₄ having a YbFe₂O₄ structure and In₂Ga₂ZnO₇ having aYb₂Fe₃O₇ structure, and the In—Ga—Zn—O film can have any of deformedstructures of the foregoing structures (M. Nakamura, N. Kimizuka, and T.Mohri, “The Phase Relations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350°C.”, J. Solid State Chem., 1991, Vol. 93, pp. 298-315). Note that alayer containing Yb is denoted by an A layer and a layer containing Feis denoted by a B layer, below. The YbFe₂O₄ structure is a repeatedstructure of ABB|ABB|ABB. As an example of a deformed structure of theYbFe₂O₄ structure, a repeated structure of ABBB|ABBB can be given.Further, the Yb₂Fe₃O₇ structure is a repeated structure ofABB|AB|ABB|AB. As an example of a deformed structure of the Yb₂Fe₃O₇structure, a repeated structure of ABBB|ABB|ABBB|ABB|ABBB|ABB| can begiven.

Note that the above metal oxide containing nitrogen at a concentrationhigher than or equal to 1×10¹⁷/cm³ and lower than 5×10¹⁹/cm³ may be usedfor the first oxide semiconductor film 103 a.

Note that the energy gap of a metal oxide which can form the first oxidesemiconductor film 103 a is 2 eV or more, preferably 2.5 eV or more,further preferably 3 eV or more. In this manner, the off-state currentof a transistor can be reduced by using an oxide semiconductor having awide energy gap.

The second oxide semiconductor film 103 b is formed using an oxidesemiconductor film which can have the second crystal structure byheating. The oxide semiconductor film which can have the second crystalstructure is easily crystallized by heat treatment and has highcrystallinity as compared to the oxide semiconductor film which can havethe trigonal and/or hexagonal first crystal structure.

The second oxide semiconductor film 103 b can be formed using zincoxide, an oxynitride semiconductor, or the like. The oxynitridesemiconductor can be obtained by adding nitrogen to any of the metaloxides listed for the first oxide semiconductor film 103 a at aconcentration higher than or equal to 5×10¹⁹/cm³ and lower than 7 at. %.

The second oxide semiconductor film 103 b is used as a seed for crystalgrowth of the first oxide semiconductor film 103 a. Therefore, thesecond oxide semiconductor film 103 b may have a thickness with whichcrystal growth is possible, typically greater than or equal to athickness of one atomic layer and less than or equal to 10 nm,preferably greater than or equal to 2 nm and less than or equal to 5 nm.When the second oxide semiconductor film 103 b is thin, throughput indeposition treatment and heat treatment can be improved.

The first oxide semiconductor film 103 a and the second oxidesemiconductor film 103 b can each be formed by a sputtering method, acoating method, a printing method, a pulsed laser evaporation method, orthe like. When the first oxide semiconductor film 103 a and the secondoxide semiconductor film 103 b are formed by a sputtering method, one ofan AC sputtering apparatus, a DC sputtering apparatus, and an RFsputtering apparatus is used.

When the second oxide semiconductor film 103 b is formed by a sputteringmethod with the use of an oxynitride semiconductor, the oxynitridesemiconductor can be deposited by changing the kind of gas introducedinto the sputtering apparatus, that is, by introducing nitrogen afterthe first oxide semiconductor film 103 a is formed. In other words, itis possible to form the first oxide semiconductor film 103 a and thesecond oxide semiconductor film 103 b successively, which is highlyproductive.

Next, first heat treatment is performed. The temperature of the firstheat treatment is higher than or equal to 150° C. and lower than orequal to 650° C., preferably higher than or equal to 200° C. and lowerthan or equal to 500° C. In addition, heating time of the first heattreatment is longer than or equal to 1 minute and shorter than or equalto 24 hours. After the temperature of the first heat treatment isgradually increased, the temperature may be set constant. When the rateat which the temperature is raised from a temperature higher than orequal to 500° C. is higher than or equal to 0.5° C./h and lower than orequal to 3° C./h, crystal growth of the second oxide semiconductor film103 b is gradually carried out; thus, the crystallinity can be furtherenhanced.

The first heat treatment is preferably performed in a rare gas(typically argon) atmosphere, an oxygen atmosphere, a nitrogenatmosphere, a dry air atmosphere, a mixed atmosphere of a rare gas(typically argon) and oxygen, or a mixed atmosphere of a rare gas andnitrogen. Specifically, a high-purity gas atmosphere is preferably used,in which the concentration of impurities such as hydrogen is reduced toapproximately several parts per million (ppm) or several parts perbillion (ppb).

A heat treatment apparatus used for the first heat treatment is notlimited to a particular apparatus, and the apparatus may be providedwith a device for heating an object to be processed by heat radiation orheat conduction from a heating element such as a resistance heatingelement. For example, an electric furnace, or a rapid thermal annealing(RTA) apparatus such as a gas rapid thermal annealing (GRTA) apparatusor a lamp rapid thermal annealing (LRTA) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas.

The first heat treatment allows crystal growth to begin from a surfaceof the second oxide semiconductor film 103 b toward the first oxidesemiconductor film 103 a. Since the second oxide semiconductor film 103b is easily crystallized, the whole second oxide semiconductor film 103b is crystallized to be an oxide semiconductor film 104 b having thesecond crystal structure. Further, since crystal growth proceeds fromthe surface of the second oxide semiconductor film 103 b toward thefirst oxide semiconductor film 103 a, a c-axis-aligned crystal region isformed. That is, the oxide semiconductor film 104 b having the secondcrystal structure includes bonds that form a hexagonal shape in an upperplane in the a-b plane. In addition, layers including hexagonal bondsare stacked and bonded in the thickness direction (the c-axisdirection), so that c-axis alignment is obtained.

When the first heat treatment is continued, crystal growth of the firstoxide semiconductor film 103 a proceeds from the interface with theoxide semiconductor film 104 b having the second crystal structuretoward the oxide insulating film 102 with the use of the oxidesemiconductor film 104 b having the second crystal structure as a seed.Crystals in the oxide semiconductor film 104 b having the second crystalstructure are aligned in the c-axis direction; therefore, by using theoxide semiconductor film 104 b having the second crystal structure as aseed, crystal in the first oxide semiconductor film 103 a can grow so asto be generally aligned with the crystal axis of the oxide semiconductorfilm 104 b having the second crystal structure. That is, crystals in thefirst oxide semiconductor film 103 a can grow while being aligned withthe c-axis. That is, an oxide semiconductor film 104 a having the firstcrystal structure includes bonds that form a hexagonal shape in an upperplane in the a-b plane. In addition, layers including hexagonal bondsare stacked and bonded in the thickness direction (the c-axisdirection), so that c-axis alignment is obtained. Through the abovesteps, the oxide semiconductor film 104 a having the c-axis-alignedfirst crystal structure can be formed (see FIG. 2B).

In the case where crystal growth proceeds perpendicularly from thesurface of the second oxide semiconductor film 103 b by the first heattreatment, the c-axes of the oxide semiconductor film 104 a having thefirst crystal structure and the oxide semiconductor film 104 b havingthe second crystal structure are generally perpendicular to the surface.

In addition, by the first heat treatment, hydrogen contained in thefirst oxide semiconductor film 103 a and the second oxide semiconductorfilm 103 b is released (i.e., dehydrogenation or dehydration occurs) andpart of oxygen contained in the oxide insulating film 102 is diffused tothe first oxide semiconductor film 103 a, the second oxide semiconductorfilm 103 b, and a region of the oxide insulating film 102 which is inthe vicinity of the interface with the first oxide semiconductor film103 a. By this step, oxygen defects included in the first oxidesemiconductor film 103 a and the second oxide semiconductor film 103 bcan be reduced; moreover, diffusion of oxygen to the region of the oxideinsulating film 102 in the vicinity of the first oxide semiconductorfilm 103 a allows defects at the interface between the oxide insulatingfilm 102 and the first oxide semiconductor film 103 a to be reduced. Asa result, the oxide semiconductor film 104 a having the first crystalstructure and the oxide semiconductor film 104 b having the secondcrystal structure, in which the hydrogen concentration and oxygendefects are reduced, can be formed.

By setting the leakage rate of a treatment chamber of the sputteringapparatus to 1×10⁻¹⁰ Pa·m³/s or lower at the time of forming the firstoxide semiconductor film 103 a and the second oxide semiconductor film103 b by a sputtering method, entry of an impurity such as an alkalimetal or hydrogen into the first oxide semiconductor film 103 a and thesecond oxide semiconductor film 103 b can be suppressed during theformation by a sputtering method. Further, with the use of an entrapmentvacuum pump (e.g., a cryopump) as an evacuation system, counter flow ofan impurity such as an alkali metal or hydrogen from the evacuationsystem can be reduced.

Further, the first oxide semiconductor film 103 a and the second oxidesemiconductor film 103 b may be formed in the state where a gasintroduced into the treatment chamber of the sputtering apparatus, suchas a nitrogen gas, an oxygen gas, or an argon gas, is heated.Consequently, the content of hydrogen in the first oxide semiconductorfilm 103 a and the second oxide semiconductor film 103 b can be reduced.

Further, before the first oxide semiconductor film 103 a and the secondoxide semiconductor film 103 b are formed by a sputtering method,preheat treatment may be performed in order to remove moisture orhydrogen contained in the sputtering apparatus or the surface or insideof a target. Consequently, the content of hydrogen in the first oxidesemiconductor film 103 a and the second oxide semiconductor film 103 bcan be reduced.

Through the above steps, the oxide semiconductor film 104 a having thefirst crystal structure and the oxide semiconductor film 104 b havingthe second crystal structure can be formed. If hydrogen is contained inthe oxide semiconductor, part thereof serves as a donor to generate anelectron as a carrier. In addition, an oxygen defect in the oxidesemiconductor also serves as a donor to generate an electron as acarrier. Therefore, when the hydrogen concentration and oxygen defectsare reduced in the oxide semiconductor film 104 a having the firstcrystal structure and the oxide semiconductor film 104 b having thesecond crystal structure, the carrier concentration in the oxidesemiconductor can be reduced and thus negative shift of the thresholdvoltage of the transistor to be manufactured later can be suppressed.

<Hexagonal Crystal Structure>

Here, a hexagonal crystal structure will be described below.

First, the c-axis-aligned second crystal structure will be describedwith reference to FIGS. 3A and 3B. As for the c-axis-aligned secondcrystal structure, FIG. 3A shows a structure in the a-b plane seen fromthe c-axis direction, and FIG. 3B shows a structure where the c-axisdirection is the vertical direction.

Examples of crystal having the second crystal structure include crystalof zinc oxide, indium nitride, and gallium nitride. Further, an oxidesemiconductor containing nitrogen, that is, an oxynitride semiconductorcan be a film having the c-axis-aligned second crystal structure in somecases.

Specifically, an In—Ga—Zn—O film containing nitrogen at a concentrationhigher than or equal to 5×10¹⁹/cm³, preferably higher than or equal to1×10²⁰/cm³ and lower than 20 at. %, becomes a film having thec-axis-aligned second crystal structure, and has one layer containing Gaand Zn between an In—O crystal plane (a crystal plane containing indiumand oxygen) and another In—O crystal plane (a crystal plane containingindium and oxygen).

Next, the c-axis-aligned hexagonal first crystal structure will bedescribed.

For example, an In—Ga—Zn—O film containing nitrogen at a concentrationhigher than or equal to 1×10¹⁷/cm³ and lower than 5×10¹⁹/cm³ becomes afilm having the c-axis-aligned hexagonal first crystal structure. TheIn—Ga—Zn—O film having the c-axis-aligned hexagonal first crystalstructure has an In—O crystal plane (a crystal plane containing indiumand oxygen) in the a-b plane and two layers containing Ga and Zn betweenIn—O crystal planes. Note that as for the two layers containing Ga andZn, there is no limitation on the position of Ga and Zn as long as atleast one of Ga and Zn is contained in each of the layers.

The second crystal structure and the first crystal structure are bothhexagonal crystal structures in which atoms are arranged in a hexagonalshape in the a-b plane. Further, the hexagonal first crystal structureis in contact with the second crystal structure, and the hexagonal firstcrystal structure is aligned with the second crystal structure.

FIGS. 4A to 4C show a manner in which the c-axis-aligned hexagonalsecond crystal structure is aligned on the c-axis-aligned first crystalstructure having the same lattice constant. FIG. 4A shows ac-axis-aligned hexagonal second crystal structure 2000, and FIG. 4Bshows a c-axis-aligned first crystal structure 2001. In addition, FIG.4C is a schematic view showing a manner in which the hexagonal secondcrystal structure 2000 is in contact with the first crystal structure2001 and the hexagonal first crystal structure 2001 is aligned with thesecond crystal structure 2000.

In this manner, the hexagonal first crystal structure 2001 is in contactwith the second crystal structure 2000 and the hexagonal first crystalstructure 2001 is aligned with the second crystal structure 2000. Thatis, a layer including the c-axis-aligned second crystal structure 2000which has high crystallinity and is easily crystallized is formed as aseed crystal layer, and an oxide semiconductor film is formed in contactwith the seed crystal layer, whereby the second crystal structure 2000included in the seed crystal layer facilitates crystallization of theoxide semiconductor film.

<Seed Crystal Layer>

Next, a seed crystal layer will be described. The seed crystal layerincludes the c-axis-aligned second crystal structure. In particular, theseed crystal layer is formed using a material that has highcrystallinity and is easily crystallized as compared to the oxidesemiconductor film.

The c-axis-aligned second crystal structure which can be applied to theseed crystal layer will be described below.

As examples of a compound which has the c-axis-aligned second crystalstructure and can be used for the seed crystal layer, zinc oxide, indiumnitride, and gallium nitride can be given. An oxide semiconductorcontaining nitrogen at a concentration higher than or equal to5×10¹⁹/cm³, preferably higher than or equal to 1×10²⁰/cm³ and lower than7 at. %, can be a film including the c-axis-aligned second crystalstructure in some cases.

In the case of using an oxide semiconductor containing nitrogen for theseed crystal layer, nitrogen is intentionally contained so that thenitrogen concentration becomes higher than or equal to 5×10¹⁹/cm³,preferably higher than or equal to 1×10²⁰/cm³ and lower than 7 at. %. Anoxide semiconductor film in which nitrogen is intentionally contained inthis range has a smaller energy gap than an oxide semiconductor film inwhich nitrogen is not contained intentionally, and thus carriers easilyflow therein.

Note that a diffraction image where bright points appear alternately maybe observed in an observation image of the c-axis-aligned second crystalstructure, which is obtained using a high-angle annular dark field(HAADF)-STEM.

FIG. 5A shows a HAADF-STEM observation image obtained by calculationbased on the c-axis-aligned second crystal structure.

FIG. 5B shows a HAADF-STEM observation image of an In—Ga—Zn—O filmformed using a deposition gas containing only nitrogen.

From each of the HAADF-STEM observation images in FIGS. 5A and 5B, itcan be confirmed that the c-axis-aligned second crystal structure has atwo-cycle layer structure.

Note that the In—Ga—Zn—O film containing nitrogen was formed by asputtering method over a quartz glass substrate to a thickness of 300nm. Deposition was performed under conditions where a target containingIn, Ga, and Zn at 1:1:1 [atomic ratio] was used, the distance betweenthe substrate and the target was 60 mm, a DC power source was used, thepower was 0.5 kw, and the pressure was 0.4 Pa. In addition, thesubstrate temperature during deposition was 400° C., and only nitrogenwas introduced as a sputtering gas into a deposition chamber at a flowrate of 40 sccm.

<Oxide Semiconductor Film>

Next, an oxide semiconductor film will be described. The oxidesemiconductor film is non-single-crystal and is not entirely in anamorphous state. The oxide semiconductor film includes at least thec-axis-aligned hexagonal first crystal structure and crystal which hasanisotropically grown from the seed crystal layer. Since the oxidesemiconductor film is not entirely in an amorphous state, formation ofan amorphous portion whose electric characteristics are unstable issuppressed.

The c-axis-aligned first crystal structure having anisotropy which canbe applied to the oxide semiconductor film will be described.

As examples of the hexagonal first crystal structure, a YbFe₂O₄structure, a Yb₂Fe₃O₇ structure, and deformed structures of theforegoing structures can be given. For example, In—Ga—Zn—O that is athree-component metal oxide has the hexagonal first crystal structureand can be used for the oxide semiconductor film. Note that theIn—Ga—Zn—O film which can be used as the oxide semiconductor film maycontain nitrogen at a concentration higher than or equal to 1×10¹⁷/cm³and lower than or equal to 5×10¹⁹/cm³.

Examples of In—Ga—Zn—O that is a three-component metal oxide includeInGaZnO₄ having a YbFe₂O₄ structure and In₂Ga₂ZnO₇ having a Yb₂Fe₃O₇structure, and the In—Ga—Zn—O can have any of deformed structures of theforegoing structures, which is disclosed in the following document: M.Nakamura, N. Kimizuka, and T. Mohri, “The Phase Relations in theIn₂O₃—Ga₂ZnO₄—ZnO System at 1350° C.”, J. Solid State Chem., 1991, Vol.93, pp. 298-315.

Further, as the oxide semiconductor film, a four-component metal oxidesuch as an In—Sn—Ga—Zn—O film; a three-component metal oxide such as anIn—Ga—Zn—O film, an In—Sn—Zn—O film, an In—Al—Zn—O film, a Sn—Ga—Zn—Ofilm, an Al—Ga—Zn—O film, or a Sn—Al—Zn—O film; a two-component metaloxide such as an In—Zn—O film, a Sn—Zn—O film, an Al—Zn—O film, or anIn—Ga—O film; or the like can be used. Further, silicon may be containedin the above oxide semiconductor film. In this specification, forexample, an In—Ga—Zn—O film means an oxide film containing indium (In),gallium (Ga), and zinc (Zn).

Crystal in the oxide semiconductor film grows anisotropically from theseed crystal layer. Accordingly, a highly crystalline region of thesemiconductor film having a hetero structure can be in contact with aninsulating surface, and interface states due to dangling bonds can bereduced, so that a semiconductor film which has a hetero structure and afavorable interface condition can be provided.

Note that a diffraction pattern where one bright spot appears everythree spots may be observed in an observation image of thec-axis-aligned hexagonal first crystal structure, which is obtainedusing a high-angle annular dark field (HAADF)-STEM.

FIG. 6A shows a HAADF-STEM observation image obtained by calculationbased on the c-axis-aligned hexagonal first crystal structure.

FIG. 6B shows a HAADF-STEM observation image of an In—Ga—Zn—O film.

From each of the HAADF-STEM observation images in FIGS. 6A and 6B, itcan be confirmed that one bright spot appears every three spots and thatthe c-axis-aligned hexagonal first crystal structure has a nine-cyclelayer structure.

Note that the In—Ga—Zn—O film was formed by a sputtering method over aquartz glass substrate to a thickness of 300 nm. Deposition wasperformed under conditions where a target containing In, Ga, and Zn at1:1:1 [atomic ratio] was used, the distance between the substrate andthe target was 60 mm, a DC power source was used, the power was 0.5 kw,and the pressure was 0.4 Pa. In addition, the substrate temperatureduring deposition was 400° C., and only oxygen was introduced as asputtering gas into a deposition chamber at a flow rate of 40 sccm.

Next, a mask is formed over the oxide semiconductor film 104 b havingthe second crystal structure, and then the oxide semiconductor film 104a having the first crystal structure and the oxide semiconductor film104 b having the second crystal structure are selectively etched usingthe mask, so that the oxide semiconductor film 105 a having the firstcrystal structure and the oxide semiconductor film 105 b having thesecond crystal structure are formed. Note that the oxide semiconductorfilm 105 a having the first crystal structure and the oxidesemiconductor film 105 b having the second crystal structure arecollectively referred to as the oxide semiconductor stack 105. Afterthat, the mask is removed.

A mask used for etching of the oxide semiconductor film 104 a having thefirst crystal structure and the oxide semiconductor film 104 b havingthe second crystal structure can be formed through a photolithographyprocess or by an inkjet method, a printing method, or the like asappropriate. In addition, the oxide semiconductor film 104 a having thefirst crystal structure and the oxide semiconductor film 104 b havingthe second crystal structure can be etched by wet etching or dry etchingas appropriate.

Next, the pair of electrodes 106 is formed in contact with the oxidesemiconductor stack 105. Then, the gate insulating film 107 is formedover the oxide insulating film 102, the oxide semiconductor stack 105,and the pair of electrodes 106. After that, the gate electrode 108 isformed over the gate insulating film 107. The insulating film 109 may beformed over the gate insulating film 107 and the gate electrode 108 (seeFIG. 2C).

The pair of electrodes 106 functions as a source electrode and a drainelectrode.

The pair of electrodes 106 can be formed using a metal element selectedfrom aluminum, chromium, copper, tantalum, titanium, molybdenum, andtungsten; an alloy containing any of these metal elements as acomponent; an alloy containing any of these metal elements incombination; or the like. Further, one or more metal elements selectedfrom manganese, magnesium, zirconium, and beryllium may be used. Inaddition, the pair of electrodes 106 can have a single-layer structureor a stacked-layer structure having two or more layers. For example, asingle-layer structure of an aluminum film containing silicon, atwo-layer structure in which a titanium film is stacked over an aluminumfilm, a two-layer structure in which a titanium film is stacked over atitanium nitride film, a two-layer structure in which a tungsten film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a tantalum nitride film, and a three-layerstructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in this order can be given. Alternatively, a film, analloy film, or a nitride film which contains aluminum and one or moreelements selected from titanium, tantalum, tungsten, molybdenum,chromium, neodymium, and scandium may be used. In the case where copperis used as a material for the pair of electrodes 106, acopper-magnesium-aluminum alloy layer may be provided in contact withthe oxide semiconductor stack 105, and a copper layer may be stacked incontact with the copper-magnesium-aluminum alloy layer.

The pair of electrodes 106 can be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible to employ a stacked-layer structure formedusing the above light-transmitting conductive material and the abovemetal element.

The pair of electrodes 106 is formed by a printing method or an inkjetmethod. Alternatively, after a conductive film is formed by a sputteringmethod, a CVD method, an evaporation method, or the like, a mask isformed over the conductive film and the conductive film is etched, andthereby the pair of electrodes 106 is formed. The mask formed over theconductive film can be formed by a printing method, an inkjet method, ora photolithography method as appropriate.

Note that the oxide semiconductor stack 105 and the pair of electrodes106 can be formed in the following manner. After a conductive film isformed over the oxide semiconductor film 104 b having the second crystalstructure, a concavo-convex shaped mask is formed using a multi-tonephotomask. The oxide semiconductor film 104 a having the first crystalstructure, the oxide semiconductor film 104 b having the second crystalstructure, and the conductive film are etched using the mask. Then, theconcavo-convex shaped mask is divided by ashing. The conductive film isselectively etched using the separated masks. In this process, thenumber of photomasks and the number of steps in the photolithographyprocess can be reduced.

The gate insulating film 107 can be formed to have a single-layerstructure or a stacked-layer structure using any of a silicon oxidefilm, a silicon oxynitride film, a silicon nitride film, a siliconnitride oxide film, an aluminum oxide film, an aluminum oxynitride film,and a gallium oxide film. It is preferable that a portion in the gateinsulating film 107, which is in contact with the oxide semiconductorstack 105, contain oxygen. It is further preferable that the gateinsulating film 107 be formed using an oxide insulating film from whichcontained oxygen is released by heating, which is similar to the oxideinsulating film 102. The use of a silicon oxide film makes diffusion ofoxygen to the oxide semiconductor stack 105 possible; thus, favorablecharacteristics can be obtained.

When a high-k material film such as a hafnium silicate (HfSiO_(x)) film,a film of hafnium silicate to which nitrogen is added(HfSi_(x)O_(y)N_(z)), a film of hafnium aluminate to which nitrogen isadded (HfAl_(x)O_(y)N_(z)), a hafnium oxide film, or an yttrium oxidefilm is used as the gate insulating film 107, gate leakage current canbe reduced. Further, a stacked-layer structure in which a high-kmaterial film and one or more of a silicon oxide film, a siliconoxynitride film, a silicon nitride film, a silicon nitride oxide film,an aluminum oxide film, an aluminum oxynitride film, and a gallium oxidefilm are stacked can be used. The thickness of the gate insulating film107 is preferably greater than or equal to 1 nm and less than or equalto 300 nm, further preferably greater than or equal to 5 nm and lessthan or equal to 50 nm.

The gate insulating film 107 is formed by a sputtering method, a CVDmethod, or the like.

Before the gate insulating film 107 is formed, the surface of the oxidesemiconductor stack 105 may be exposed to plasma of an oxidizing gassuch as oxygen, ozone, or dinitrogen monoxide so as to be oxidized,thereby reducing oxygen defects.

The gate electrode 108 can be formed using a metal element selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, andtungsten; an alloy containing any of these metal elements as acomponent; an alloy containing any of these metal elements incombination; or the like. Further, one or more metal elements selectedfrom manganese, magnesium, zirconium, and beryllium may be used. Inaddition, the gate electrode 108 can have a single-layer structure or astacked-layer structure having two or more layers. For example, asingle-layer structure of an aluminum film containing silicon, atwo-layer structure in which a titanium film is stacked over an aluminumfilm, a two-layer structure in which a titanium film is stacked over atitanium nitride film, a two-layer structure in which a tungsten film isstacked over a titanium nitride film, a two-layer structure in which atungsten film is stacked over a tantalum nitride film, and a three-layerstructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in this order can be given. Alternatively, a film, analloy film, or a nitride film which contains aluminum and one or moreelements selected from titanium, tantalum, tungsten, molybdenum,chromium, neodymium, and scandium may be used.

The gate electrode 108 can be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. It is also possible to employ a stacked-layer structure formedusing the above light-transmitting conductive material and the abovemetal element.

As a material layer in contact with the gate insulating film, anIn—Ga—Zn—O film containing nitrogen, an In—Sn—O film containingnitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O filmcontaining nitrogen, a Sn—O film containing nitrogen, an In—O filmcontaining nitrogen, or a film of a metal nitride (such as InN or ZnN)is preferably provided between the gate electrode 108 and the gateinsulating film. These films each have a work function of 5 eV orhigher, preferably 5.5 eV or higher; thus, the threshold voltage of theelectric characteristics of the transistor can be positive. Accordingly,a so-called normally-off switching element can be realized. For example,in the case of using an In—Ga—Zn—O film containing nitrogen, anIn—Ga—Zn—O film having a nitrogen concentration at least higher thanthat of the oxide semiconductor stack 105 is used; specifically, anIn—Ga—Zn—O film having a nitrogen concentration of 7 at. % or higher isused.

The gate electrode 108 is formed by a printing method or an inkjetmethod. Alternatively, after a conductive film is formed by a sputteringmethod, a CVD method, an evaporation method, or the like, a mask isformed over the conductive film and the conductive film is etched, andthereby the gate electrode 108 is formed. The mask formed over theconductive film can be formed by a printing method, an inkjet method, ora photolithography method as appropriate.

The insulating film 109 can be formed as appropriate by using any of theinsulating films listed for the gate insulating film 107. When a siliconnitride film is formed as the insulating film 109 by a sputteringmethod, entry of moisture and an alkali metal from the outside can beprevented, and thus the number of impurities included in the oxidesemiconductor stack 105 can be reduced.

Note that, after formation of the gate insulating film 107 or theinsulating film 109, heat treatment (temperature range: higher than orequal to 150° C. and lower than or equal to 650° C., preferably higherthan or equal to 200° C. and lower than or equal to 500° C.) may beperformed in an atmosphere which contains little hydrogen and moisture(such as a nitrogen atmosphere, an oxygen atmosphere, or a dry airatmosphere (in terms of moisture, for example, the dew point is lowerthan or equal to −40° C., preferably lower than or equal to −60° C.)).

Through the above steps, a transistor whose channel includes an oxidesemiconductor stack including crystal which has hexagonal bonds in thea-b plane and a c-axis-aligned trigonal and/or hexagonal structure canbe manufactured.

The oxide semiconductor stack described in this embodiment has highcrystallinity and evenness in a region in the vicinity of the interfacewith the gate insulating film and thus has stable electriccharacteristics; accordingly, a highly reliable transistor can beobtained. The oxide semiconductor stack including crystal which hashexagonal bonds in the a-b plane and a c-axis-aligned trigonal and/orhexagonal structure is used for a channel region of a transistor,whereby a transistor in which the amount of change in the thresholdvoltage between before and after light irradiation or a bias-temperaturestress (BT) test performed on the transistor is small and which hasstable electric characteristics can be manufactured.

Embodiment 2

In this embodiment, a structure of a transistor which is different fromthat in Embodiment 1 and a manufacturing method thereof will bedescribed with reference to FIGS. 7A and 7B and FIGS. 8A to 8C. Thisembodiment is different from Embodiment 1 in that a pair of electrodesis provided between an oxide insulating film and an oxide semiconductorstack. Note that FIG. 7B corresponds to a cross-sectional view alongdashed-dotted line C-D in FIG. 7A which is a top view. In FIG. 7A, thesubstrate 101, the oxide insulating film 102, a gate insulating film117, and an insulating film 119 are not illustrated. FIGS. 8A to 8C arecross-sectional views illustrating a manufacturing process of thetransistor illustrated in FIG. 7B.

The transistor illustrated in FIG. 7B includes the oxide insulating film102 formed over the substrate 101; a pair of electrodes 116 which isformed over the oxide insulating film 102 and functions as a sourceelectrode and a drain electrode; an oxide semiconductor stack 115 whichcovers the oxide insulating film 102 and the pair of electrodes 116functioning as the source electrode and the drain electrode; the gateinsulating film 117 formed over the oxide insulating film 102, the pairof electrodes 116, and the oxide semiconductor stack 115; and a gateelectrode 118 which overlaps with the oxide semiconductor stack 115 withthe gate insulating film 117 positioned therebetween. Further, theinsulating film 119 which covers the gate insulating film 117 and thegate electrode 118 may be provided. Furthermore, a pair of wirings 120may be provided in contact with the pair of electrodes 116 in openingsin the insulating film 119.

The oxide semiconductor stack 115 is characterized in that an oxidesemiconductor film 115 a having a first crystal structure, which is incontact with the oxide insulating film 102 and the pair of electrodes116, and an oxide semiconductor film 115 b having a second crystalstructure, which is in contact with the oxide semiconductor film 115 ahaving the first crystal structure, are stacked.

Further, the oxide semiconductor stack 115 is characterized in thatcrystal growth has occurred in the oxide semiconductor film 115 a havingthe first crystal structure with the use of the oxide semiconductor film115 b having the second crystal structure as seed crystal.

As in Embodiment 1, both the oxide semiconductor film having the secondcrystal structure and the oxide semiconductor film having the firstcrystal structure include trigonal and/or hexagonal crystal; therefore,a hexagonal lattice image can be observed from the c-axis direction.

Note that each of the oxide semiconductor film 115 a having the firstcrystal structure and the oxide semiconductor film 115 b having thesecond crystal structure is non-single-crystal, is not entirely in anamorphous state, and includes c-axis-aligned crystals.

Next, a method for manufacturing the transistor in FIG. 7B will bedescribed with reference to FIGS. 8A to 8C.

As illustrated in FIG. 8A, the oxide insulating film 102 is formed overthe substrate 101 as in Embodiment 1. Next, the pair of electrodes 116is formed over the oxide insulating film 102. Then, a first oxidesemiconductor film 113 a and a second oxide semiconductor film 113 b areformed over the pair of electrodes 116 and the oxide insulating film102.

The pair of electrodes 116 can be formed as appropriate by using amaterial and a formation method which are similar to those of the pairof electrodes 106 described in Embodiment 1.

The first oxide semiconductor film 113 a and the second oxidesemiconductor film 113 b can be formed as appropriate by using materialsand formation methods which are similar to those of the first oxidesemiconductor film 103 a and the second oxide semiconductor film 103 bdescribed in Embodiment 1.

Next, in a manner similar to that in Embodiment 1, first heat treatmentis performed. The first heat treatment allows crystal growth to beginfrom a surface of the second oxide semiconductor film 113 b toward thefirst oxide semiconductor film 113 a, so that the second oxidesemiconductor film 113 b becomes an oxide semiconductor film 114 bhaving the second crystal structure. The oxide semiconductor film 114 bhaving the second crystal structure includes c-axis-aligned crystal.

When the first heat treatment is continued, crystal growth of the firstoxide semiconductor film 113 a proceeds from the interface with theoxide semiconductor film 114 b having the second crystal structuretoward the oxide insulating film 102 with the use of the oxidesemiconductor film 114 b having the second crystal structure as a seed,so that an oxide semiconductor film 114 a having the first crystalstructure is formed. The oxide semiconductor film 114 a having the firstcrystal structure includes c-axis-aligned crystal (see FIG. 8B).

Through the above steps, the oxide semiconductor film 114 a having thefirst crystal structure and the oxide semiconductor film 114 b havingthe second crystal structure can be formed.

Next, a mask is formed over the oxide semiconductor film 114 b havingthe second crystal structure, and then the oxide semiconductor film 114a having the first crystal structure and the oxide semiconductor film114 b having the second crystal structure are selectively etched usingthe mask, so that the oxide semiconductor film 115 a having the firstcrystal structure and the oxide semiconductor film 115 b having thesecond crystal structure are formed. Note that the oxide semiconductorfilm 115 a having the first crystal structure and the oxidesemiconductor film 115 b having the second crystal structure arecollectively referred to as the oxide semiconductor stack 115. Afterthat, the mask is removed.

Next, the gate insulating film 117 is formed over the oxide insulatingfilm 102, the pair of electrodes 116, and the oxide semiconductor stack115. Then, the gate electrode 118 is formed over the gate insulatingfilm 117.

After that, the insulating film 119 is formed over the gate insulatingfilm 117 and the gate electrode 118. Then, after a mask is formed overthe insulating film 119, the gate insulating film 117 and the insulatingfilm 119 are partly etched to form openings. Then, the wirings 120 whichare connected to the pair of electrodes 116 through the openings may beformed (see FIG. 8C).

The gate insulating film 117 can be formed as appropriate by using amaterial and a formation method which are similar to those of the gateinsulating film 107 described in Embodiment 1.

The gate electrode 118 can be formed as appropriate by using a materialand a formation method which are similar to those of the gate electrode108 described in Embodiment 1.

The insulating film 119 can be formed as appropriate by using a materialand a formation method which are similar to those of the insulating film109 described in Embodiment 1.

The wirings 120 can be formed as appropriate by using a material and aformation method which are similar to those of the pair of electrodes116.

Through the above steps, a transistor whose channel region includes anoxide semiconductor stack including crystal which has hexagonal bonds inthe a-b plane and a c-axis-aligned trigonal and/or hexagonal structurecan be manufactured.

The oxide semiconductor stack described in this embodiment has highcrystallinity and evenness in a region in the vicinity of the interfacewith the gate insulating film and thus has stable electriccharacteristics; accordingly, a highly reliable transistor can beobtained. The oxide semiconductor stack including crystal which hashexagonal bonds in the a-b plane and a c-axis-aligned trigonal and/orhexagonal structure is used for a channel region of a transistor,whereby a transistor in which the amount of change in the thresholdvoltage between before and after light irradiation or a bias-temperaturestress (BT) test performed on the transistor is small and which hasstable electric characteristics can be manufactured.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 3

In this embodiment, a transistor in which an oxide semiconductor film isused for a channel and a manufacturing method thereof will be describedwith reference to FIGS. 9A and 9B and FIGS. 10A to 10E. FIG. 9B is across-sectional view illustrating a structure of a transistor which isone embodiment of a structure of a semiconductor device, and correspondsto a cross-sectional view along dashed-dotted line A-B in FIG. 9A whichis a top view. Note that in FIG. 9A, the substrate 101, the oxideinsulating film 102, the gate insulating film 107, and the insulatingfilm 109 are not illustrated. FIGS. 10A to 10E are cross-sectional viewsillustrating a manufacturing process of the transistor illustrated inFIG. 9B.

The transistor illustrated in FIG. 9B includes the oxide insulating film102 formed over the substrate 101; the oxide semiconductor stack 105formed over the oxide insulating film 102; the pair of electrodes 106which is formed over the oxide semiconductor stack 105 and functions asa source electrode and a drain electrode; the gate insulating film 107formed over the oxide insulating film 102, the oxide semiconductor stack105, and the pair of electrodes 106; and the gate electrode 108 whichoverlaps with the oxide semiconductor stack 105 with the gate insulatingfilm 107 positioned therebetween. Further, the insulating film 109 whichcovers the gate insulating film 107 and the gate electrode 108 may beprovided.

The oxide semiconductor stack 105 is characterized in that the oxidesemiconductor film 105 a having a first crystal structure, which is incontact with the oxide insulating film 102; the oxide semiconductor film105 b having a second crystal structure, which is in contact with theoxide semiconductor film 105 a having the first crystal structure; andan oxide semiconductor film 105 c having a third crystal structure,which is in contact with the oxide semiconductor film 105 b having thesecond crystal structure and the gate insulating film 107, are stacked.

That is, the oxide semiconductor film 105 a having the first crystalstructure and the oxide semiconductor film 105 c having the thirdcrystal structure are provided under and over the oxide semiconductorfilm 105 b having the second crystal structure.

Further, the oxide semiconductor stack 105 is characterized in thatcrystal growth has occurred in each of the oxide semiconductor film 105a having the first crystal structure and the oxide semiconductor film105 c having the third crystal structure with the use of the oxidesemiconductor film 105 b having the second crystal structure as seedcrystal.

The crystal structures of the oxide semiconductor film 105 a having thefirst crystal structure and the oxide semiconductor film 105 c havingthe third crystal structure are each a trigonal and/or hexagonal crystalstructure and any one of a YbFe₂O₄ structure, a Yb₂Fe₃O₇ structure, anda non-wurtzite structure. Note that the non-wurtzite structure is acrystal structure which is not a trigonal and/or hexagonal wurtzitetype.

Further, the crystal structure of the oxide semiconductor film 105 bhaving the second crystal structure is a wurtzite structure which is oneof trigonal and/or hexagonal crystal structures.

In other words, since all of the oxide semiconductor film having thefirst crystal structure, the oxide semiconductor film having the secondcrystal structure, and the oxide semiconductor film having the thirdcrystal structure include trigonal and/or hexagonal crystal, a hexagonallattice image can be observed from the c-axis direction.

Note that each of the oxide semiconductor film 105 a having the firstcrystal structure, the oxide semiconductor film 105 b having the secondcrystal structure, and the oxide semiconductor film 105 c having thethird crystal structure is non-single-crystal, is not entirely in anamorphous state, and includes a c-axis-aligned crystal region. That is,each of the oxide semiconductor films has an amorphous region and ac-axis-aligned crystal region.

Next, a method for manufacturing the transistor in FIG. 9B will bedescribed with reference to FIGS. 10A to 10E.

As illustrated in FIG. 10A, in a manner similar to that in Embodiment 1,after the oxide insulating film 102 is formed over the substrate 101,the first oxide semiconductor film 103 a is formed over the oxideinsulating film 102, and the second oxide semiconductor film 103 b isformed over the first oxide semiconductor film 103 a.

The oxide insulating film 102 is formed using an oxide insulating filmfrom which part of contained oxygen is released by heating. The oxideinsulating film from which part of contained oxygen is released byheating is preferably an oxide insulating film which contains oxygen atan amount exceeding the amount of oxygen in its stoichiometriccomposition. With the oxide insulating film from which part of containedoxygen is released by heating, oxygen can be diffused to the first oxidesemiconductor film 103 a and the second oxide semiconductor film 103 bby heating. Typical examples of the oxide insulating film 102 includefilms of silicon oxide, silicon oxynitride, silicon nitride oxide,aluminum oxide, aluminum oxynitride, gallium oxide, hafnium oxide, andyttrium oxide.

The thickness of the oxide insulating film 102 is greater than or equalto 50 nm, preferably greater than or equal to 200 nm and less than orequal to 500 nm. With the use of the thick oxide insulating film 102,the amount of oxygen released from the oxide insulating film 102 can beincreased, and defects at the interface between the oxide insulatingfilm 102 and an oxide semiconductor film to be formed later can bereduced.

The oxide insulating film 102 is formed by a sputtering method, a CVDmethod, or the like. The oxide insulating film from which part ofcontained oxygen is released by heating is easily formed by a sputteringmethod, which is preferable.

When the oxide insulating film from which part of contained oxygen isreleased by heating is formed by a sputtering method, the amount ofoxygen in a deposition gas is preferably large, and oxygen, a mixed gasof oxygen and a rare gas, or the like can be used. Typically, the oxygenconcentration in the deposition gas is preferably higher than or equalto 6% and lower than or equal to 100%.

The first oxide semiconductor film 103 a is formed using an oxidesemiconductor film which can include trigonal and/or hexagonal crystaland have any one crystal structure of a non-wurtzite structure, aYbFe₂O₄ structure, a Yb₂Fe₃O₇ structure, and deformed structures of theforegoing structures by heating.

As an example of the oxide semiconductor film having the first crystalstructure, an In—Ga—Zn—O film that is a three-component metal oxideincludes trigonal and/or hexagonal non-wurtzite crystal. In addition,examples of the In—Ga—Zn—O film that is a three-component metal oxideinclude InGaZnO₄ having a YbFe₂O₄ structure and In₂Ga₂ZnO₇ having aYb₂Fe₃O₇ structure, and the In—Ga—Zn—O film can have any of deformedstructures of the foregoing structures (M. Nakamura, N. Kimizuka, and T.Mohri, “The Phase Relations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350°C.”, J. Solid State Chem., 1991, Vol. 93, pp. 298-315).

As the first oxide semiconductor film 103 a, a four-component metaloxide such as an In—Sn—Ga—Zn—O film; a three-component metal oxide suchas an In—Ga—Zn—O film, an In—Sn—Zn—O film, an In—Al—Zn—O film, aSn—Ga—Zn—O film, an Al—Ga—Zn—O film, or a Sn—Al—Zn—O film; atwo-component metal oxide such as an In—Zn—O film, a Sn—Zn—O film, anAl—Zn—O film, or an In—Ga—O film; or the like can be used. Further, SiO₂may be contained in the above oxide semiconductor. In thisspecification, for example, an In—Ga—Zn—O film means an oxide filmcontaining indium (In), gallium (Ga), and zinc (Zn). Note that the abovemetal oxide containing nitrogen at a concentration higher than or equalto 1×10¹⁷/cm³ and lower than 5×10¹⁹/cm³ may be used for the first oxidesemiconductor film 103 a.

Note that the energy gap of a metal oxide which can form the first oxidesemiconductor film 103 a is 2 eV or more, preferably 2.5 eV or more,further preferably 3 eV or more. In this manner, the off-state currentof a transistor can be reduced by using an oxide semiconductor having awide energy gap.

The second oxide semiconductor film 103 b is formed using an oxidesemiconductor film which can have a wurtzite crystal structure byheating. The oxide semiconductor film which can have a wurtzite crystalstructure is easily crystallized by heat treatment and has highcrystallinity as compared to an oxide semiconductor film which can havea trigonal and/or hexagonal crystal structure.

The second oxide semiconductor film 103 b can be formed using zincoxide, an oxynitride semiconductor, or the like. The oxynitridesemiconductor can be obtained by adding nitrogen to any of the metaloxides listed for the first oxide semiconductor film 103 a at aconcentration higher than or equal to 5×10¹⁹/cm³, preferably higher thanor equal to 1×10²⁰/cm³ and lower than 7 at. %.

The second oxide semiconductor film 103 b is used as a seed for crystalgrowth of the first oxide semiconductor film 103 a and a third oxidesemiconductor film 103 c which is formed later. Therefore, the secondoxide semiconductor film 103 b may have a thickness with which crystalgrowth is possible, typically greater than or equal to a thickness ofone atomic layer and less than or equal to 10 nm, preferably greaterthan or equal to 2 nm and less than or equal to 5 nm. When the secondoxide semiconductor film 103 b is thin, throughput in depositiontreatment and heat treatment can be improved.

The first oxide semiconductor film 103 a and the second oxidesemiconductor film 103 b can each be formed by a sputtering method, acoating method, a printing method, a pulsed laser evaporation method, orthe like. When the first oxide semiconductor film 103 a and the secondoxide semiconductor film 103 b are formed by a sputtering method, one ofan AC sputtering apparatus, a DC sputtering apparatus, and an RFsputtering apparatus is used.

When the second oxide semiconductor film 103 b is formed by a sputteringmethod with the use of an oxynitride semiconductor, the oxynitridesemiconductor can be deposited by changing the kind of gas introducedinto the sputtering apparatus, that is, by introducing nitrogen afterthe first oxide semiconductor film 103 a is formed. In other words, itis possible to form the first oxide semiconductor film 103 a and thesecond oxide semiconductor film 103 b successively, which is highlyproductive.

Next, in a manner similar to that in Embodiment 1, first heat treatmentis performed.

The first heat treatment allows crystal growth to begin from a surfaceof the second oxide semiconductor film 103 b toward the first oxidesemiconductor film 103 a. Since the second oxide semiconductor film 103b is easily crystallized, the whole second oxide semiconductor film 103b is crystallized to be the oxide semiconductor film 104 b having thesecond crystal structure that is a wurtzite crystal structure. Further,since crystal growth proceeds from the surface of the second oxidesemiconductor film 103 b toward the first oxide semiconductor film 103a, a c-axis-aligned crystal region is formed. That is, the oxidesemiconductor film 104 b having the second crystal structure includesbonds that form a hexagonal shape in a plane in the a-b plane. Inaddition, layers including hexagonal bonds are stacked and bonded in thethickness direction (the c-axis direction), so that c-axis alignment isobtained.

When the first heat treatment is continued, crystal growth of the firstoxide semiconductor film 103 a proceeds from the interface with theoxide semiconductor film 104 b having the second crystal structuretoward the oxide insulating film 102 with the use of the oxidesemiconductor film 104 b having the second crystal structure as a seed.Crystals in the oxide semiconductor film 104 b having the second crystalstructure are c-axis aligned; therefore, by using the oxidesemiconductor film 104 b having the second crystal structure as a seed,crystal in the first oxide semiconductor film 103 a can grow so as to begenerally aligned with the crystal axis of the oxide semiconductor film104 b having the second crystal structure. That is, crystals in thefirst oxide semiconductor film 103 a can grow while being aligned withthe c-axis. That is, the oxide semiconductor film 104 a having the firstcrystal structure includes bonds that form a hexagonal shape in a planein the a-b plane. In addition, layers including hexagonal bonds arestacked and bonded in the thickness direction (the c-axis direction), sothat c-axis alignment is obtained. Through the above steps, the oxidesemiconductor film 104 a having the c-axis-aligned first crystalstructure can be formed (see FIG. 10B).

In the case where crystal growth proceeds perpendicularly from thesurface of the second oxide semiconductor film 103 b by the first heattreatment, the c-axes of the oxide semiconductor film 104 a having thefirst crystal structure and the oxide semiconductor film 104 b havingthe second crystal structure are generally perpendicular to the surface.

In addition, by the first heat treatment, hydrogen contained in thefirst oxide semiconductor film 103 a and the second oxide semiconductorfilm 103 b is released (i.e., dehydrogenation or dehydration occurs) andpart of oxygen contained in the oxide insulating film 102 is diffused tothe first oxide semiconductor film 103 a, the second oxide semiconductorfilm 103 b, and a region of the oxide insulating film 102 which is inthe vicinity of the interface with the first oxide semiconductor film103 a. By this step, oxygen defects included in the first oxidesemiconductor film 103 a and the second oxide semiconductor film 103 bcan be reduced; moreover, diffusion of oxygen to the region of the oxideinsulating film 102 in the vicinity of the first oxide semiconductorfilm 103 a allows defects at the interface between the oxide insulatingfilm 102 and the first oxide semiconductor film 103 a to be reduced. Asa result, the oxide semiconductor film 104 a having the first crystalstructure and the oxide semiconductor film 104 b having the secondcrystal structure, in which the hydrogen concentration and oxygendefects are reduced, can be formed.

Next, as illustrated in FIG. 10C, the third oxide semiconductor film 103c is formed over the oxide semiconductor film 104 b having the secondcrystal structure. The third oxide semiconductor film 103 c can beformed by using a material and a formation method which are similar tothose of the first oxide semiconductor film 103 a. The thickness of thethird oxide semiconductor film 103 c may be determined as appropriate bya practitioner in accordance with a device to be manufactured. Forexample, the total thickness of the first oxide semiconductor film 103a, the second oxide semiconductor film 103 b, and the third oxidesemiconductor film 103 c can be greater than or equal to 10 nm and lessthan or equal to 200 nm.

By setting the leakage rate of a treatment chamber of the sputteringapparatus to 1×10⁻¹⁰ Pa·m³/s or lower at the time of forming one or moreof the first oxide semiconductor film 103 a, the second oxidesemiconductor film 103 b, and the third oxide semiconductor film 103 cby a sputtering method, entry of an impurity such as an alkali metal orhydrogen into the first oxide semiconductor film 103 a, the second oxidesemiconductor film 103 b, and the third oxide semiconductor film 103 ccan be suppressed during the formation by a sputtering method. Further,with the use of an entrapment vacuum pump (e.g., a cryopump) as anevacuation system, counter flow of an impurity such as an alkali metalor hydrogen from the evacuation system can be reduced.

Further, one or more of the first oxide semiconductor film 103 a, thesecond oxide semiconductor film 103 b, and the third oxide semiconductorfilm 103 c may be formed in the state where a gas introduced into thetreatment chamber of the sputtering apparatus, such as a nitrogen gas,an oxygen gas, or an argon gas, is heated. Consequently, the content ofhydrogen in one or more of the first oxide semiconductor film 103 a, thesecond oxide semiconductor film 103 b, and the third oxide semiconductorfilm 103 c can be reduced.

Further, before one or more of the first oxide semiconductor film 103 a,the second oxide semiconductor film 103 b, and the third oxidesemiconductor film 103 c are formed by a sputtering method, preheattreatment may be performed in order to remove moisture or hydrogencontained in the sputtering apparatus or the surface or inside of atarget. Consequently, the content of hydrogen in one or more of thefirst oxide semiconductor film 103 a, the second oxide semiconductorfilm 103 b, and the third oxide semiconductor film 103 c can be reduced.

Next, second heat treatment is performed. The temperature of the secondheat treatment is higher than or equal to 150° C. and lower than orequal to 650° C., preferably higher than or equal to 200° C. and lowerthan or equal to 500° C. In addition, heating time of the second heattreatment is longer than or equal to 1 minute and shorter than or equalto 24 hours.

The second heat treatment can be performed in an atmosphere similar tothat of the first heat treatment. In addition, a heating apparatussimilar to that of the first heat treatment can be used as appropriatefor the second heat treatment.

The second heat treatment allows crystal growth to begin from the oxidesemiconductor film 104 b having the second crystal structure that is awurtzite crystal structure toward the third oxide semiconductor film 103c. Crystals in the oxide semiconductor film 104 b having the secondcrystal structure are c-axis aligned; therefore, by using the oxidesemiconductor film 104 b having the second crystal structure as a seed,crystals in the third oxide semiconductor film 103 c can grow so as tobe generally aligned with the crystal axis of the oxide semiconductorfilm 104 b having the second crystal structure as in the case of thefirst oxide semiconductor film 103 a. That is, crystals in the thirdoxide semiconductor film 103 c can grow while being aligned with thec-axis. That is, an oxide semiconductor film 104 c having the thirdcrystal structure includes bonds that form a hexagonal shape in a planein the a-b plane. In addition, layers including hexagonal bonds arestacked and bonded in the thickness direction (the c-axis direction), sothat c-axis alignment is obtained. Through the above steps, the oxidesemiconductor film 104 c having the c-axis-aligned third crystalstructure can be formed. Moreover, since crystal growth occurs with theuse of the oxide semiconductor film 104 b having the second crystalstructure as a seed, crystal growth of the third oxide semiconductorfilm 103 c is enhanced, so that a surface of the oxide semiconductorfilm 104 c having the third crystal structure has high evenness as wellas high crystallinity (see FIG. 10D).

In the case where crystal growth proceeds perpendicularly from thesurface of the oxide semiconductor film 104 b having the second crystalstructure by the second heat treatment, the c-axis of the oxidesemiconductor film 104 c having the third crystal structure is generallyperpendicular to the surface of the oxide semiconductor film 104 bhaving the second crystal structure.

Furthermore, by the second heat treatment, hydrogen contained in thethird oxide semiconductor film 103 c is released (i.e., dehydrogenationor dehydration occurs) as in the case of the first heat treatment. As aresult, the oxide semiconductor film 104 c having the third crystalstructure, in which the hydrogen concentration is reduced, can beformed.

Through the above steps, the oxide semiconductor film 104 a having thefirst crystal structure, the oxide semiconductor film 104 b having thesecond crystal structure, and the oxide semiconductor film 104 c havingthe third crystal structure can be formed; note that the first to thirdcrystal structures are trigonal and/or hexagonal crystal structures. Thehydrogen concentration and oxygen defects in the oxide semiconductorfilm 104 a having the first crystal structure, the oxide semiconductorfilm 104 b having the second crystal structure, and the oxidesemiconductor film 104 c having the third crystal structure can bereduced. If hydrogen is contained in the oxide semiconductor, partthereof serves as a donor to generate an electron as a carrier. Inaddition, an oxygen defect in the oxide semiconductor also serves as adonor to generate an electron as a carrier. Therefore, when the hydrogenconcentration and oxygen defects are reduced in the oxide semiconductorfilm 104 a having the first crystal structure, the oxide semiconductorfilm 104 b having the second crystal structure, and the oxidesemiconductor film 104 c having the third crystal structure, the carrierconcentration in the oxide semiconductor can be reduced and thusnegative shift of the threshold voltage of the transistor to bemanufactured later can be suppressed. For those reasons, reduction inthe hydrogen concentration and the number of oxygen defects in the oxidesemiconductor film 104 a having the first crystal structure, the oxidesemiconductor film 104 b having the second crystal structure, and theoxide semiconductor film 104 c having the third crystal structure leadsto suppression of negative shift of the threshold voltage of thetransistor to be manufactured later.

Next, in a manner similar to that in Embodiment 1, a mask is formed overthe oxide semiconductor film 104 c having the third crystal structure,and then the oxide semiconductor film 104 a having the first crystalstructure, the oxide semiconductor film 104 b having the second crystalstructure, and the oxide semiconductor film 104 c having the thirdcrystal structure are selectively etched using the mask, so that theoxide semiconductor film 105 a having the first crystal structure, theoxide semiconductor film 105 b having the second crystal structure, andthe oxide semiconductor film 105 c having the third crystal structureare formed. Note that the oxide semiconductor film 105 a having thefirst crystal structure, the oxide semiconductor film 105 b having thesecond crystal structure, and the oxide semiconductor film 105 c havingthe third crystal structure are collectively referred to as the oxidesemiconductor stack 105. After that, the mask is removed.

Next, the pair of electrodes 106 is formed in contact with the oxidesemiconductor stack 105. Then, the gate insulating film 107 is formedover the oxide insulating film 102, the oxide semiconductor stack 105,and the pair of electrodes 106. After that, the gate electrode 108 isformed over the gate insulating film 107. The insulating film 109 may beformed over the gate insulating film 107 and the gate electrode 108 (seeFIG. 10E).

The pair of electrodes 106 can be formed as appropriate by using amaterial and a formation method which are similar to those of the pairof electrodes 106 described in Embodiment 1.

Note that the oxide semiconductor stack 105 and the pair of electrodes106 can be formed in the following manner. After a conductive film isformed over the oxide semiconductor film 104 c having the third crystalstructure, a concavo-convex shaped mask is formed using a multi-tonephotomask. The oxide semiconductor film 104 a having the first crystalstructure, the oxide semiconductor film 104 b having the second crystalstructure, the oxide semiconductor film 104 c having the third crystalstructure, and the conductive film are etched using the mask. Then, theconcavo-convex shaped mask is separated by ashing. The conductive filmis selectively etched using the separated masks. In this process, thenumber of photomasks and the number of steps in the photolithographyprocess can be reduced.

The gate insulating film 107 can be formed as appropriate by using amaterial and a formation method which are similar to those of the gateinsulating film 107 described in Embodiment 1.

Before the gate insulating film 107 is formed, the surface of the oxidesemiconductor stack 105 may be exposed to plasma of an oxidizing gassuch as oxygen, ozone, or dinitrogen monoxide so as to be oxidized,thereby reducing oxygen defects.

The gate electrode 108 can be formed as appropriate by using a materialand a formation method which are similar to those of the gate electrode108 described in Embodiment 1.

Note that, after formation of the gate insulating film 107 or theinsulating film 109, heat treatment (temperature range: higher than orequal to 150° C. and lower than or equal to 650° C., preferably higherthan or equal to 200° C. and lower than or equal to 500° C.) may beperformed in an atmosphere which contains little hydrogen and moisture(such as a nitrogen atmosphere, an oxygen atmosphere, or a dry airatmosphere (in terms of moisture, for example, the dew point is lowerthan or equal to −40° C., preferably lower than or equal to −60° C.)).

Through the above steps, a transistor whose channel includes an oxidesemiconductor stack including a crystal region which has hexagonal bondsin the a-b plane and a c-axis-aligned trigonal and/or hexagonalstructure can be manufactured.

The oxide semiconductor stack described in this embodiment has highcrystallinity and evenness in a region in the vicinity of the interfacewith the gate insulating film and thus has stable electriccharacteristics; accordingly, a highly reliable transistor can beobtained. The oxide semiconductor stack including a crystal region whichhas hexagonal bonds in the a-b plane and a c-axis-aligned trigonaland/or hexagonal structure is used for a channel region of a transistor,whereby a transistor in which the amount of change in the thresholdvoltage between before and after light irradiation or a bias-temperaturestress (BT) test performed on the transistor is small and which hasstable electric characteristics can be manufactured.

Note that an oxynitride semiconductor has a smaller energy gap than anoxide semiconductor, and thus carriers easily flow therein. Therefore,by reducing the thickness of the oxide semiconductor film 105 c havingthe third crystal structure in the transistor, a buried channeltransistor in which the oxide semiconductor film 105 b having the secondcrystal structure serves as a channel is obtained. As a result, atransistor which has favorable electric characteristics without aninfluence of the condition of the interface between the gate insulatingfilm 107 and the oxide semiconductor film 105 c having the third crystalstructure can be manufactured.

Embodiment 4

In this embodiment, a structure of a transistor which is different fromthat in Embodiment 3 and a manufacturing method thereof will bedescribed with reference to FIGS. 11A and 11B and FIGS. 12A to 12D. Thisembodiment is different from Embodiment 3 in that a pair of electrodesis provided between an oxide insulating film and an oxide semiconductorstack. Note that FIG. 11B corresponds to a cross-sectional view alongdashed-dotted line C-D in FIG. 11A which is a top view. In FIG. 11A, thesubstrate 101, the oxide insulating film 102, the gate insulating film117, and the insulating film 119 are not illustrated. FIGS. 12A to 12Dare cross-sectional views illustrating a manufacturing process of thetransistor illustrated in FIG. 11B.

The transistor illustrated in FIG. 11B includes the oxide insulatingfilm 102 formed over the substrate 101; the pair of electrodes 116 whichis formed over the oxide insulating film 102 and functions as a sourceelectrode and a drain electrode; the oxide semiconductor stack 115 whichcovers the oxide insulating film 102 and the pair of electrodes 116functioning as the source electrode and the drain electrode; the gateinsulating film 117 formed over the oxide insulating film 102, the pairof electrodes 116, and the oxide semiconductor stack 115; and the gateelectrode 118 which overlaps with the oxide semiconductor stack 115 withthe gate insulating film 117 positioned therebetween. Further, theinsulating film 119 which covers the gate insulating film 117 and thegate electrode 118 may be provided. Furthermore, the pair of wirings 120may be provided in contact with the pair of electrodes 116 in openingsin the insulating film 119.

The oxide semiconductor stack 115 is characterized in that the oxidesemiconductor film 115 a having a first crystal structure, which is incontact with the oxide insulating film 102 and the pair of electrodes116; the oxide semiconductor film 115 b having a second crystalstructure, which is in contact with the oxide semiconductor film 115 ahaving the first crystal structure; and an oxide semiconductor film 115c having a third crystal structure, which is in contact with the oxidesemiconductor film 115 b having the second crystal structure and thegate insulating film 117, are stacked.

That is, the oxide semiconductor film 115 a having the first crystalstructure and the oxide semiconductor film 115 c having the thirdcrystal structure are provided under and over the oxide semiconductorfilm 115 b having the second crystal structure.

Further, the oxide semiconductor stack 115 is characterized in thatcrystal growth has occurred in each of the oxide semiconductor film 115a having the first crystal structure and the oxide semiconductor film115 c having the third crystal structure with the use of the oxidesemiconductor film 115 b having the second crystal structure as seedcrystal.

The crystal structures of the oxide semiconductor film 115 a having thefirst crystal structure and the oxide semiconductor film 115 c havingthe third crystal structure are each a trigonal and/or hexagonal crystalstructure and any one of a non-wurtzite structure, a YbFe₂O₄ structure,a Yb₂Fe₃O₇ structure, and deformed structures of the foregoingstructures. Note that the non-wurtzite structure is a crystal structurewhich is not a trigonal and/or hexagonal wurtzite type.

Further, the crystal structure of the oxide semiconductor film 115 bhaving the second crystal structure is a wurtzite structure which is oneof trigonal and/or hexagonal crystal structures.

As in Embodiment 3, since all of the oxide semiconductor film 115 ahaving the first crystal structure, the oxide semiconductor film 115 bhaving the second crystal structure, and the oxide semiconductor film115 c having the third crystal structure include trigonal and/orhexagonal crystal, a hexagonal lattice image can be observed from thec-axis direction.

Note that each of the oxide semiconductor film 115 a having the firstcrystal structure, the oxide semiconductor film 115 b having the secondcrystal structure, and the oxide semiconductor film 115 c having thethird crystal structure is non-single-crystal, is not entirely in anamorphous state, and includes a c-axis-aligned crystal region. That is,each of the oxide semiconductor films has an amorphous region and ac-axis-aligned crystal region.

Next, a method for manufacturing the transistor in FIG. 11B will bedescribed with reference to FIGS. 12A to 12D.

As illustrated in FIG. 12A, the oxide insulating film 102 is formed overthe substrate 101 as in Embodiment 1. Next, the pair of electrodes 116is formed over the oxide insulating film 102. Then, the first oxidesemiconductor film 113 a and the second oxide semiconductor film 113 bare formed over the pair of electrodes 116 and the oxide insulating film102.

The pair of electrodes 116 can be formed as appropriate by using amaterial and a formation method which are similar to those of the pairof electrodes 106 described in Embodiment 1.

The first oxide semiconductor film 113 a and the second oxidesemiconductor film 113 b can be formed as appropriate by using materialsand formation methods which are similar to those of the first oxidesemiconductor film 103 a and the second oxide semiconductor film 103 bdescribed in Embodiment 1.

Next, in a manner similar to that in Embodiment 1, first heat treatmentis performed. The first heat treatment allows crystal growth to beginfrom a surface of the second oxide semiconductor film 113 b toward thefirst oxide semiconductor film 113 a, so that the second oxidesemiconductor film 113 b becomes the oxide semiconductor film 114 bhaving the second crystal structure that is a wurtzite crystalstructure. The oxide semiconductor film 114 b having the second crystalstructure includes c-axis-aligned crystal.

When the first heat treatment is continued, crystal growth of the firstoxide semiconductor film 113 a proceeds from the interface with theoxide semiconductor film 114 b having the second crystal structuretoward the oxide insulating film 102 with the use of the oxidesemiconductor film 114 b having the second crystal structure as a seed,so that the oxide semiconductor film 114 a having the first crystalstructure is formed. The oxide semiconductor film 114 a having the firstcrystal structure includes a c-axis-aligned crystal region.

Next, a third oxide semiconductor film 113 c is formed over the oxidesemiconductor film 114 b having the second crystal structure (see FIG.12B). The third oxide semiconductor film 113 c can be formed asappropriate by using a material and a formation method which are similarto those of the third oxide semiconductor film 103 c described inEmbodiment 3.

Next, in a manner similar to that in Embodiment 3, second heat treatmentis performed. The second heat treatment allows crystal growth to beginfrom the interface with the oxide semiconductor film 114 b having thesecond crystal structure that is a wurtzite crystal structure toward thethird oxide semiconductor film 113 c, so that the third oxidesemiconductor film 113 c becomes an oxide semiconductor film 114 chaving the third crystal structure. The oxide semiconductor film 114 chaving the third crystal structure includes a c-axis-aligned crystalregion (see FIG. 12C).

Through the above steps, the oxide semiconductor film 114 a having thefirst crystal structure, the oxide semiconductor film 114 b having thesecond crystal structure, and the oxide semiconductor film 114 c havingthe third crystal structure can be formed; note that the first to thirdcrystal structures are trigonal and/or hexagonal crystal structures.

Next, a mask is formed over the oxide semiconductor film 114 c havingthe third crystal structure, and then the oxide semiconductor film 114 ahaving the first crystal structure, the oxide semiconductor film 114 bhaving the second crystal structure, and the oxide semiconductor film114 c having the third crystal structure are selectively etched usingthe mask, so that the oxide semiconductor film 115 a having the firstcrystal structure, the oxide semiconductor film 115 b having the secondcrystal structure, and the oxide semiconductor film 115 c having thethird crystal structure are formed. Note that the oxide semiconductorfilm 115 a having the first crystal structure, the oxide semiconductorfilm 115 b having the second crystal structure, and the oxidesemiconductor film 115 c having the third crystal structure arecollectively referred to as the oxide semiconductor stack 115. Afterthat, the mask is removed.

Next, the gate insulating film 117 is formed over the oxide insulatingfilm 102, the pair of electrodes 116, and the oxide semiconductor stack115. Then, the gate electrode 118 is formed over the gate insulatingfilm 117.

After that, the insulating film 119 is formed over the gate insulatingfilm 117 and the gate electrode 118. Then, after a mask is formed overthe insulating film 119, the gate insulating film 117 and the insulatingfilm 119 are partly etched to form openings. Then, the wirings 120 whichare connected to the pair of electrodes 116 through the openings may beformed (see FIG. 12D).

The gate insulating film 117 can be formed as appropriate by using amaterial and a formation method which are similar to those of the gateinsulating film 107 described in Embodiment 1.

The gate electrode 118 can be formed as appropriate by using a materialand a formation method which are similar to those of the gate electrode108 described in Embodiment 1.

The insulating film 119 can be formed as appropriate by using a materialand a formation method which are similar to those of the insulating film109 described in Embodiment 1.

The wirings 120 can be formed as appropriate by using a material and aformation method which are similar to those of the pair of electrodes116.

Through the above steps, a transistor whose channel region includes anoxide semiconductor stack including a crystal region which has hexagonalbonds in the a-b plane and a c-axis-aligned trigonal and/or hexagonalstructure can be manufactured.

The oxide semiconductor stack described in this embodiment has highcrystallinity and evenness in a region in the vicinity of the interfacewith the gate insulating film and thus has stable electriccharacteristics; accordingly, a highly reliable transistor can beobtained. The oxide semiconductor stack including a crystal region whichhas hexagonal bonds in the a-b plane and a c-axis-aligned trigonaland/or hexagonal structure is used for a channel region of a transistor,whereby a transistor in which the amount of change in the thresholdvoltage between before and after light irradiation or a bias-temperaturestress (BT) test performed on the transistor is small and which hasstable electric characteristics can be manufactured.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 5

In this embodiment, a structure of a transistor which is different fromthe structures of the transistors in Embodiments 1 to 4 and amanufacturing method thereof will be described with reference to FIGS.13A and 13B and FIGS. 14A to 14D. This embodiment is different fromEmbodiments 1 to 4 in that a gate electrode is provided between an oxideinsulating film and a gate insulating film. That is, although top-gatetransistors are described in Embodiments 1 to 4, a bottom-gatetransistor will be described in this embodiment. Note that FIG. 13Bcorresponds to a cross-sectional view along dashed-dotted line E-F inFIG. 13A which is a top view. In FIG. 13A, the substrate 101, the oxideinsulating film 102, a gate insulating film 127, and an insulating film129 are not illustrated. FIGS. 14A to 14D are cross-sectional viewsillustrating a manufacturing process of the transistor illustrated inFIG. 13B.

The transistor illustrated in FIG. 13B includes the oxide insulatingfilm 102 formed over the substrate 101; a gate electrode 128 formed overthe oxide insulating film 102; the gate insulating film 127 which coversthe oxide insulating film 102 and the gate electrode 128; an oxidesemiconductor stack 125 which overlaps with the gate electrode 128 withthe gate insulating film 127 positioned therebetween; and a pair ofelectrodes 126 which is in contact with the oxide semiconductor stack125 and functions as a source electrode and a drain electrode. Further,the insulating film 129 which covers the gate insulating film 127, theoxide semiconductor stack 125, and the pair of electrodes 126 may beprovided.

The oxide semiconductor stack 125 is characterized in that an oxidesemiconductor film 125 b having a first crystal structure, which is incontact with the gate insulating film 127, and an oxide semiconductorfilm 125 c having a second crystal structure, which is in contact withthe oxide semiconductor film 125 b having the first crystal structure,are stacked.

Further, the oxide semiconductor stack 125 is characterized in thatcrystal growth has occurred in the oxide semiconductor film 125 c havingthe second crystal structure with the use of the oxide semiconductorfilm 125 b having the first crystal structure as seed crystal.

The oxide semiconductor film 125 b having the first crystal structurehas a wurtzite crystal structure which is one of trigonal and/orhexagonal crystal structures.

The oxide semiconductor film 125 c having the second crystal structureincludes trigonal and/or hexagonal crystal and has any one crystalstructure of a YbFe₂O₄ structure, a Yb₂Fe₃O₇ structure, and anon-wurtzite structure.

Since both the oxide semiconductor film having the first crystalstructure and the oxide semiconductor film having the second crystalstructure include trigonal and/or hexagonal crystal, a hexagonal latticeimage can be observed from the c-axis direction.

Each of the oxide semiconductor film 125 b having the first crystalstructure and the oxide semiconductor film 125 c having the secondcrystal structure is non-single-crystal, is not entirely in an amorphousstate, and includes a c-axis-aligned crystal region. That is, each ofthe oxide semiconductor films has an amorphous region and ac-axis-aligned crystal region.

Note that the oxide semiconductor stack 125 has a two-layer structureincluding the oxide semiconductor film 125 b having the first crystalstructure and the oxide semiconductor film 125 c having the secondcrystal structure, here; however, a three-layer oxide semiconductorstack may be formed as in Embodiments 3 and 4.

Next, a method for manufacturing the transistor in FIG. 13B will bedescribed with reference to FIGS. 14A to 14D.

As illustrated in FIG. 14A, the oxide insulating film 102 is formed overthe substrate 101 as in Embodiment 1. Next, the gate electrode 128 isformed over the oxide insulating film 102. Then, the gate insulatingfilm 127 is formed over the oxide insulating film 102 and the gateelectrode 128. After that, a first oxide semiconductor film 123 b isformed over the gate insulating film 127.

The gate electrode 128 and the gate insulating film 127 can be formed asappropriate by using materials and formation methods which are similarto those of the gate electrode 108 and the gate insulating film 107described in Embodiment 1.

The first oxide semiconductor film 123 b can be formed as appropriate byusing a material and a formation method which are similar to those ofthe second oxide semiconductor film 103 b described in Embodiment 1.

Next, in a manner similar to that in Embodiment 1, first heat treatmentis performed. The first heat treatment allows crystal growth to beginfrom a surface of the first oxide semiconductor film 123 b toward thegate insulating film 127, so that an oxide semiconductor film 124 bhaving the first crystal structure is formed. The oxide semiconductorfilm 124 b having the first crystal structure includes a c-axis-alignedcrystal region.

Next, a second oxide semiconductor film 123 c is formed over the oxidesemiconductor film 124 b having the first crystal structure (see FIG.14B). The second oxide semiconductor film 123 c can be formed asappropriate by using a material and a formation method which are similarto those of the third oxide semiconductor film 103 c described inEmbodiment 3.

Next, in a manner similar to that in Embodiment 3, second heat treatmentis performed. This heat treatment allows crystal growth to begin fromthe interface with the oxide semiconductor film 124 b having the firstcrystal structure toward the second oxide semiconductor film 123 c, sothat the second oxide semiconductor film 123 c becomes an oxidesemiconductor film 124 c having the second crystal structure. The oxidesemiconductor film 124 c having the second crystal structure includes ac-axis-aligned crystal region (see FIG. 14C).

Through the above steps, the oxide semiconductor film 124 b having thefirst crystal structure and the oxide semiconductor film 124 c havingthe second crystal structure can be formed.

Next, a mask is formed over the oxide semiconductor film 124 c havingthe second crystal structure, and then the oxide semiconductor film 124b having the first crystal structure and the oxide semiconductor film124 c having the second crystal structure are selectively etched usingthe mask, so that the oxide semiconductor film 125 b having the firstcrystal structure and the oxide semiconductor film 125 c having thesecond crystal structure are formed. Note that the oxide semiconductorfilm 125 b having the first crystal structure and the oxidesemiconductor film 125 c having the second crystal structure arecollectively referred to as the oxide semiconductor stack 125. Afterthat, the mask is removed.

Next, in a manner similar to that in Embodiment 1, the pair ofelectrodes 126 is formed.

Next, the insulating film 129 may be formed over the gate insulatingfilm 127, the pair of electrodes 126, and the oxide semiconductor stack125 (see FIG. 14D).

The insulating film 129 can be formed as appropriate by using a materialand a formation method which are similar to those of the insulating film109 described in Embodiment 1.

Through the above steps, a transistor whose channel region includes anoxide semiconductor stack including a crystal region which has hexagonalbonds in the a-b plane and a c-axis-aligned trigonal and/or hexagonalstructure can be manufactured.

Note that a channel-etched transistor is described in this embodiment;however, this embodiment can be applied to a channel protectivetransistor.

The oxide semiconductor stack has high crystallinity and evenness in aregion in the vicinity of the interface with the gate insulating filmand thus has stable electric characteristics; accordingly, a highlyreliable transistor can be obtained. The oxide semiconductor stackincluding crystal which has hexagonal bonds in the a-b plane and ac-axis-aligned trigonal and/or hexagonal structure is used for a channelregion of a transistor, whereby a transistor in which the amount ofchange in the threshold voltage between before and after lightirradiation or a bias-temperature stress (BT) test performed on thetransistor is small and which has stable electric characteristics can bemanufactured.

Note that an oxynitride semiconductor has a smaller energy gap than anoxide semiconductor, and thus carriers easily flow therein. Therefore,by forming the oxide semiconductor film 125 b having the first crystalstructure, which is in contact with the gate insulating film 127, withthe use of an oxynitride semiconductor film, a transistor havingfavorable electric characteristics can be manufactured.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 6

In this embodiment, a structure of a transistor which is different fromthe structures of the transistors in Embodiments 1 to 5 and amanufacturing method thereof will be described with reference to FIGS.15A and 15B and FIGS. 16A to 16D. In this embodiment, a bottom-gatetransistor will be described. The transistor is different from that inEmbodiment 5 in that a pair of electrodes is provided between a gateinsulating film and an oxide semiconductor stack. Note that FIG. 15Bcorresponds to a cross-sectional view along dashed-dotted line G-H inFIG. 15A which is a top view. In FIG. 15A, the substrate 101, the oxideinsulating film 102, a gate insulating film 137, and an insulating film139 are not illustrated. FIGS. 16A to 16D are cross-sectional viewsillustrating a manufacturing process of the transistor illustrated inFIG. 15B.

The transistor illustrated in FIG. 15B includes the oxide insulatingfilm 102 formed over the substrate 101; a gate electrode 138 formed overthe oxide insulating film 102; the gate insulating film 137 which coversthe oxide insulating film 102 and the gate electrode 138; a pair ofelectrodes 136 which functions as a source electrode and a drainelectrode; and an oxide semiconductor stack 135 which is in contact withthe gate insulating film 137 and the pair of electrodes 136. Further,the insulating film 139 which covers the gate insulating film 137, theoxide semiconductor stack 135, and the pair of electrodes 136 may beprovided.

The oxide semiconductor stack 135 is characterized in that an oxidesemiconductor film 135 b having a first crystal structure, which is incontact with the gate insulating film 137, and an oxide semiconductorfilm 135 c having a second crystal structure, which is in contact withthe oxide semiconductor film 135 b having the first crystal structure,are stacked.

Further, the oxide semiconductor stack 135 is characterized in thatcrystal growth has occurred in the oxide semiconductor film 135 c havingthe second crystal structure with the use of the oxide semiconductorfilm 135 b having the first crystal structure as seed crystal.

The oxide semiconductor film 135 b having the first crystal structurehas a wurtzite crystal structure which is one of trigonal and/orhexagonal crystal structures.

The oxide semiconductor film 135 c having the second crystal structureincludes trigonal and/or hexagonal crystal and has any one crystalstructure of a YbFe₂O₄ structure, a Yb₂Fe₃O₇ structure, and anon-wurtzite structure.

Since both the oxide semiconductor film having the first crystalstructure and the oxide semiconductor film having the second crystalstructure include trigonal and/or hexagonal crystal, a hexagonal latticeimage can be observed from the c-axis direction.

Each of the oxide semiconductor film 135 b having the first crystalstructure and the oxide semiconductor film 135 c having the secondcrystal structure is non-single-crystal, is not entirely in an amorphousstate, and includes a c-axis-aligned crystal region. That is, each ofthe oxide semiconductor films has an amorphous region and ac-axis-aligned crystal region.

Note that the oxide semiconductor stack 135 has a two-layer structureincluding the oxide semiconductor film 135 b having the first crystalstructure and the oxide semiconductor film 135 c having the secondcrystal structure, here; however, a three-layer oxide semiconductorstack may be formed as in Embodiments 3 and 4.

Next, a method for manufacturing the transistor in FIG. 15B will bedescribed with reference to FIGS. 16A to 16D.

As illustrated in FIG. 16A, the oxide insulating film 102 is formed overthe substrate 101 as in Embodiment 1. Next, the gate electrode 138 isformed over the oxide insulating film 102. Then, the gate insulatingfilm 137 is formed over the oxide insulating film 102 and the gateelectrode 138. After that, the pair of electrodes 136 is formed over thegate insulating film 137. Then, a first oxide semiconductor film 133 bis formed over the gate insulating film 137 and the pair of electrodes136.

The gate electrode 138, the gate insulating film 137, and the firstoxide semiconductor film 133 b can be formed as appropriate by usingmaterials and formation methods which are similar to those of the gateelectrode 108, the gate insulating film 107, and the second oxidesemiconductor film 103 b described in Embodiment 3.

Next, in a manner similar to that in Embodiment 1, first heat treatmentis performed. The first heat treatment allows crystal growth to beginfrom a surface of the first oxide semiconductor film 133 b toward thegate insulating film 137, so that the first oxide semiconductor film 133b becomes an oxide semiconductor film 134 b having the first crystalstructure. The oxide semiconductor film 134 b having the first crystalstructure includes a c-axis-aligned crystal region.

Next, a second oxide semiconductor film 133 c is formed over the oxidesemiconductor film 134 b having the first crystal structure (see FIG.16B). The second oxide semiconductor film 133 c can be formed asappropriate by using a material and a formation method which are similarto those of the third oxide semiconductor film 103 c described inEmbodiment 3.

Next, in a manner similar to that in Embodiment 3, second heat treatmentis performed. This heat treatment allows crystal growth to begin fromthe interface with the oxide semiconductor film 134 b having the firstcrystal structure toward the second oxide semiconductor film 133 c, sothat the second oxide semiconductor film 133 c becomes an oxidesemiconductor film 134 c having the second crystal structure. The oxidesemiconductor film 134 c having the second crystal structure includes ac-axis-aligned crystal region (see FIG. 16C).

Through the above steps, the oxide semiconductor film 134 b having thefirst crystal structure and the oxide semiconductor film 134 c havingthe second crystal structure can be formed.

Next, a mask is formed over the oxide semiconductor film 134 c havingthe second crystal structure, and then the oxide semiconductor film 134b having the first crystal structure and the oxide semiconductor film134 c having the second crystal structure are selectively etched usingthe mask, so that the oxide semiconductor film 135 b having the firstcrystal structure and the oxide semiconductor film 135 c having thesecond crystal structure are formed. Note that the oxide semiconductorfilm 135 b having the first crystal structure and the oxidesemiconductor film 135 c having the second crystal structure arecollectively referred to as the oxide semiconductor stack 135. Afterthat, the mask is removed.

Next, the insulating film 139 may be formed over the oxide insulatingfilm 102, the pair of electrodes 136, and the oxide semiconductor stack135 (see FIG. 16D).

The insulating film 139 can be formed as appropriate by using a materialand a formation method which are similar to those of the insulating film109 described in Embodiment 3.

Through the above steps, a transistor whose channel region includes anoxide semiconductor stack including crystal which has hexagonal bonds inthe a-b plane and a c-axis-aligned trigonal and/or hexagonal structurecan be manufactured.

Note that a channel-etched transistor is described in this embodiment;however, this embodiment can be applied to a channel protectivetransistor.

The oxide semiconductor stack has high crystallinity and evenness in aregion in the vicinity of the interface with the gate insulating filmand thus has stable electric characteristics; accordingly, a highlyreliable transistor can be obtained. The oxide semiconductor stackincluding a crystal region which has hexagonal bonds in the a-b planeand a c-axis-aligned trigonal and/or hexagonal structure is used for achannel region of a transistor, whereby a transistor in which the amountof change in the threshold voltage between before and after lightirradiation or a bias-temperature stress (BT) test performed on thetransistor is small and which has stable electric characteristics can bemanufactured.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 7

In this embodiment, the case where the transistor described in any ofEmbodiments 1 to 6 has a plurality of gate electrodes will be described.Although the transistor described in Embodiment 5 is used in thisembodiment, this embodiment can be applied to the transistors describedin Embodiments 1 to 4 and Embodiment 6 as appropriate.

In a manner similar to that in Embodiment 5, the oxide insulating film102 is formed over the substrate 101, and a first gate electrode 148 aand a first gate insulating film 147 a are formed over the oxideinsulating film 102 as illustrated in FIG. 17. Then, the oxidesemiconductor stack 125 in which the oxide semiconductor film 125 bhaving a first crystal structure and the oxide semiconductor film 125 chaving a second crystal structure are stacked, the pair of electrodes126, and a second gate insulating film 147 b are formed over the firstgate insulating film 147 a.

Next, a second gate electrode 148 b is formed over the second gateinsulating film 147 b in a region overlapping with the oxidesemiconductor stack 125. The insulating film 129 may be formed over thesecond gate insulating film 147 b and the second gate electrode 148 b asa protective film.

The first gate electrode 148 a and the second gate electrode 148 b canbe formed in a manner similar to that of the gate electrode 108described in Embodiment 1.

The first gate insulating film 147 a and the second gate insulating film147 b can be formed in a manner similar to that of the gate insulatingfilm 107 described in Embodiment 1.

The first gate electrode 148 a and the second gate electrode 148 b maybe connected. In this case, the first gate electrode 148 a and thesecond gate electrode 148 b have the same potential and channel regionsare formed on the first gate electrode 148 a side and the second gateelectrode 148 b side of the oxide semiconductor stack 125, and therebythe on-state current and field effect mobility of the transistor can beincreased.

Alternatively, it is also possible that the first gate electrode 148 aand the second gate electrode 148 b are not connected and supplied withdifferent potentials. In this case, the threshold voltage of thetransistor can be controlled.

In this embodiment, the pair of electrodes 126 is formed between theoxide semiconductor stack 125 and the second gate insulating film 147 b,but the pair of electrodes 126 may be formed between the first gateinsulating film 147 a and the oxide semiconductor stack 125.

Through the above steps, a transistor having a plurality of gateelectrodes can be manufactured.

Embodiment 8

In this embodiment, an embodiment will be described below, in which adisplay device including at least part of a driver circuit and atransistor disposed in a pixel portion are provided over one substrateis manufactured.

A transistor disposed in the pixel portion is formed in accordance withany of Embodiments 1 to 7. Further, the transistor described in any ofEmbodiments 1 to 7 is an n-channel transistor, and thus part of a drivercircuit that can be formed using n-channel transistors among drivercircuits is formed over the same substrate as the transistor in thepixel portion.

FIG. 18A is one embodiment of a block diagram of an active matrixdisplay device. Over a substrate 5300 in the display device, a pixelportion 5301, a first scan line driver circuit 5302, a second scan linedriver circuit 5303, and a signal line driver circuit 5304 are provided.In the pixel portion 5301, a plurality of signal lines extended from thesignal line driver circuit 5304 is arranged and a plurality of scanlines extended from the first scan line driver circuit 5302 and thesecond scan line driver circuit 5303 is arranged. Note that pixels whichinclude display elements are provided in a matrix form in respectiveregions where the scan lines and the signal lines intersect with eachother. Further, the substrate 5300 in the display device is connected toa timing control circuit (also referred to as a controller or acontroller IC) through a connection portion such as a flexible printedcircuit (FPC).

In FIG. 18A, the first scan line driver circuit 5302, the second scanline driver circuit 5303, and the signal line driver circuit 5304 areformed over the same substrate 5300 as the pixel portion 5301.Accordingly, the number of components of a driver circuit and the likeprovided outside is reduced, so that reduction in cost can be achieved.Further, if the driver circuit is provided outside the substrate 5300,wirings would need to be extended and the number of wiring connectionswould be increased. However, if the driver circuit is provided over thesubstrate 5300, the number of wiring connections can be reduced.Consequently, improvement in reliability and yield can be achieved.

FIG. 18B illustrates one embodiment of a circuit configuration of thepixel portion. Here, a pixel structure of a VA liquid crystal displaypanel is shown.

In this pixel structure, a plurality of pixel electrodes is included inone pixel, and a transistor is connected to each of the pixelelectrodes. The transistors are driven by different gate signals. Thatis, signals that are supplied to individual pixel electrodes in amulti-domain pixel are controlled independently.

A gate wiring 602 of a transistor 628 and a gate wiring 603 of atransistor 629 are separated so that different gate signals can besupplied thereto. In contrast, a source or drain electrode 616functioning as a data line is used in common for the transistor 628 andthe transistor 629. As each of the transistors 628 and 629, any of thetransistors described in Embodiments 1 to 7 can be used as appropriate.

A first pixel electrode and a second pixel electrode have differentshapes and are separated by a slit. The second pixel electrode isprovided so as to surround the external side of the first pixelelectrode which is spread in a V shape. Timings of voltage applicationare varied between the first pixel electrode and the second pixelelectrode by the transistor 628 and the transistor 629 in order tocontrol alignment of liquid crystal. The transistor 628 is connected tothe gate wiring 602, and the transistor 629 is connected to the gatewiring 603. When different gate signals are supplied to the gate wiring602 and the gate wiring 603, operation timings of the transistor 628 andthe transistor 629 can be varied.

Further, a storage capacitor is formed using a capacitor wiring 690, agate insulating film as a dielectric, and a capacitor electrodeelectrically connected to the first pixel electrode or the second pixelelectrode.

The first pixel electrode, a liquid crystal layer, and a counterelectrode overlap with each other to form a first liquid crystal element651. The second pixel electrode, the liquid crystal layer, and thecounter electrode overlap with each other to form a second liquidcrystal element 652. The pixel structure is a multi-domain structure inwhich the first liquid crystal element 651 and the second liquid crystalelement 652 are provided in one pixel.

Note that the pixel structure is not limited to that illustrated in FIG.18B. For example, a switch, a resistor, a capacitor, a transistor, asensor, or a logic circuit may be added to the pixel illustrated in FIG.18B.

In this embodiment, an embodiment of a VA liquid crystal display panelis shown; however, one embodiment of the present invention is notparticularly limited thereto and can be applied to various modes ofliquid crystal display devices. For example, as a method for improvingviewing angle characteristics, one embodiment of the present inventioncan be applied to a lateral electric field mode (also referred to as anIPS mode) in which an electric field in the horizontal direction to themain surface of a substrate is applied to a liquid crystal layer.

For example, it is preferable to use liquid crystal exhibiting a bluephase for which an alignment film is not necessary for an IPS liquidcrystal display panel. A blue phase is one of liquid crystal phases,which appears just before a cholesteric phase changes into an isotropicphase while temperature of cholesteric liquid crystal is increased.Since the blue phase appears only in a narrow temperature range, aliquid crystal composition in which a chiral agent is mixed is used forthe liquid crystal layer of the liquid crystal element in order toimprove the temperature range. The liquid crystal composition whichincludes liquid crystal exhibiting a blue phase and a chiral agent has ashort response time of 1 millisecond or less, and has optical isotropy,which makes the alignment process unneeded and viewing angle dependencesmall.

Further, in order to improve moving-image characteristics of a liquidcrystal display device, a driving technique (e.g., a field sequentialmethod) may be employed, in which a plurality of light-emitting diodes(LEDs) or a plurality of EL light sources is used as a backlight to forma surface light source, and each light source of the surface lightsource is independently driven in a pulsed manner in one frame period.As the surface light source, three or more kinds of LEDs may be used oran LED emitting white light may be used. In the case where three or morekinds of light sources emitting different colors (e.g., light sources ofred (R), green (G), and blue (B)) are used as the surface light source,color display can be performed without a color filter. Further, in thecase where an LED emitting white light is used as the surface lightsource, color display is performed with a color filter. Since aplurality of LEDs can be controlled independently, the light emissiontiming of LEDs can be synchronized with the timing at which the liquidcrystal layer is optically modulated. The LEDs can be partly turned off,and thus power consumption can be reduced particularly in the case ofdisplaying an image in which a black display region occupies a largearea in one screen.

FIG. 18C illustrates one embodiment of a circuit configuration of thepixel portion. Here, a pixel structure of a display panel using anorganic EL element is shown.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a film containing a light-emitting organic compound, andthus current flows. The carriers (electrons and holes) are recombined,and thus the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

FIG. 18C illustrates one embodiment of a pixel structure to whichdigital time grayscale driving can be applied, as an embodiment of asemiconductor device.

A structure and operation of a pixel to which digital time grayscaledriving can be applied will be described. An embodiment is described inthis embodiment, in which one pixel includes two n-channel transistorsusing an oxide semiconductor film in a channel region.

A pixel 6400 includes a switching transistor 6401, a driving transistor6402, a light-emitting element 6404, and a capacitor 6403. A gateelectrode of the switching transistor 6401 is connected to a scan line6406. A first electrode (one of a source electrode and a drainelectrode) of the switching transistor 6401 is connected to a signalline 6405. A second electrode (the other of the source electrode and thedrain electrode) of the switching transistor 6401 is connected to a gateelectrode of the driving transistor 6402. The gate electrode of thedriving transistor 6402 is connected to a power supply line 6407 throughthe capacitor 6403. A first electrode of the driving transistor 6402 isconnected to the power supply line 6407. A second electrode of thedriving transistor 6402 is connected to a first electrode (a pixelelectrode) of the light-emitting element 6404. A second electrode of thelight-emitting element 6404 corresponds to a common electrode 6408. Thecommon electrode 6408 is electrically connected to a common potentialline provided over the same substrate.

The second electrode (the common electrode 6408) of the light-emittingelement 6404 is set to a low power supply potential. Note that the lowpower supply potential is a potential satisfying the relation, the lowpower supply potential<a high power supply potential with reference tothe high power supply potential that is supplied to the power supplyline 6407. As the low power supply potential, GND or 0 V may beemployed, for example. A potential difference between the high powersupply potential and the low power supply potential is applied to thelight-emitting element 6404 and current is supplied to thelight-emitting element 6404, so that the light-emitting element 6404emits light. Here, in order to make the light-emitting element 6404 emitlight, each potential is set so that the potential difference betweenthe high power supply potential and the low power supply potential ishigher than or equal to forward threshold voltage of the light-emittingelement 6404.

Note that gate capacitance of the driving transistor 6402 may be used asa substitute for the capacitor 6403, so that the capacitor 6403 can beomitted. The gate capacitance of the driving transistor 6402 may beformed between the channel region and the gate electrode.

In the case of a voltage-input voltage driving method, a video signal isinput to the gate electrode of the driving transistor 6402 so that thedriving transistor 6402 is either sufficiently turned on or sufficientlyturned off. That is, the driving transistor 6402 operates in a linearregion, and thus voltage higher than the voltage of the power supplyline 6407 is applied to the gate electrode of the driving transistor6402. Note that voltage higher than or equal to (voltage of the powersupply line+Vth of the driving transistor 6402) is applied to the signalline 6405.

In the case of performing analog grayscale driving instead of digitaltime grayscale driving, the same pixel structure as FIG. 18C can be usedby changing signal input.

In the case of performing analog grayscale driving, voltage higher thanor equal to the sum of the forward voltage of the light-emitting element6404 and Vth of the driving transistor 6402 is applied to the gateelectrode of the driving transistor 6402. The forward voltage of thelight-emitting element 6404 indicates voltage at which a desiredluminance is obtained, and includes at least forward threshold voltage.By inputting a video signal which enables the driving transistor 6402 tooperate in a saturation region, current can be supplied to thelight-emitting element 6404. In order for the driving transistor 6402 tooperate in the saturation region, the potential of the power supply line6407 is set to be higher than the gate potential of the drivingtransistor 6402. When an analog video signal is used, it is possible tofeed current to the light-emitting element 6404 in accordance with thevideo signal and perform analog grayscale driving.

Note that the pixel structure is not limited to that illustrated in FIG.18C. For example, a switch, a resistor, a capacitor, a sensor, atransistor, or a logic circuit may be added to the pixel illustrated inFIG. 18C.

Next, structures of a light-emitting element will be described withreference to cross-sectional structures of a pixel, which areillustrated in FIGS. 19A to 19C. Here, cross-sectional structures of apixel will be described by taking the case where a light-emittingelement driving transistor is an n-channel transistor as an example.Light-emitting element driving transistors 7011, 7021, and 7001 whichare used for semiconductor devices illustrated in FIGS. 19A to 19C canbe manufactured in a manner similar to that of the transistor describedin any of Embodiments 1 to 7.

At least one of a first electrode and a second electrode of thelight-emitting element is formed using a conductive film that transmitsvisible light, and light emission is extracted from the light-emittingelement. When attention is focused on the direction from which lightemission is extracted, the following structures can be given: a topemission structure in which light emission is extracted from the side ofa substrate on which a light-emitting element is formed without passingthrough the substrate over which the light-emitting element and atransistor are formed; a bottom emission structure in which lightemission is extracted from the side where the light-emitting element isnot formed through the substrate over which the light-emitting elementis formed; and a dual emission structure in which light emission isextracted from both the side of the substrate on which thelight-emitting element is formed and the other side of the substratethrough the substrate. The pixel configuration illustrated in FIG. 18Ccan be applied to a light-emitting element having any of these emissionstructures.

A light-emitting element having a bottom emission structure will bedescribed with reference to FIG. 19A. The light-emitting element havinga bottom emission structure emits light in the direction indicated by anarrow in FIG. 19A.

In FIG. 19A, an embodiment in which the n-channel transistor describedin Embodiment 1 is used as the light-emitting element driving transistor7011 is shown; however, one embodiment of the present invention is notparticularly limited thereto.

In FIG. 19A, an EL layer 7014 and a second electrode 7015 are stacked inthis order over a first electrode 7017 having a light-transmittingproperty, which is electrically connected to a source electrode or adrain electrode of the light-emitting element driving transistor 7011.

The first electrode 7017 is formed using a conductive film thattransmits visible light. For the conductive film that transmits visiblelight, for example, indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium tin oxide(hereinafter referred to as ITO), indium zinc oxide, or indium tin oxideto which silicon oxide is added can be used. Further, a metal thin filmhaving a thickness large enough to transmit light (preferablyapproximately 5 nm to 30 nm) can also be used. For example, an aluminumfilm with a thickness of 20 nm can be stacked over another conductivefilm having a light-transmitting property.

As for the second electrode 7015, a material which efficiently reflectslight emitted from the EL layer 7014 is preferably used, in which casethe light extraction efficiency can be improved. Note that the secondelectrode 7015 may have a stacked-layer structure. For example, aconductive film that transmits visible light, which is formed on theside in contact with the EL layer 7014, and a light-blocking film 7016may be stacked. As the light-blocking film, although a metal film or thelike which efficiently reflects light emitted from the EL layer ispreferable, a resin or the like to which a black pigment is added canalso be used, for example.

Note that one of the first electrode 7017 and the second electrode 7015functions as an anode, and the other functions as a cathode. It ispreferable to use a substance having a high work function for theelectrode which functions as an anode, and a substance having a low workfunction for the electrode which functions as a cathode.

As a material having a high work function, for example, ZrN, Ti, W, Ni,Pt, Cr, ITO, or In—Zn—O can be used. As a material having a low workfunction, an alkali metal such as Li or Cs, an alkaline earth metal suchas Mg, Ca, or Sr, an alloy containing any of these (such as Mg:Ag orAl:Li), a rare earth metal such as Yb or Er, or the like can be used.

Note that when power consumption is compared, it is preferable that thefirst electrode 7017 function as a cathode and the second electrode 7015function as an anode because increase in voltage of a driver circuitportion can be suppressed and power consumption can be reduced.

The EL layer 7014 includes at least a light-emitting layer and may beeither a single layer or a stack of plural layers. As the structure inwhich a plurality of layers is stacked, a structure in which an anode, ahole-injection layer, a hole-transport layer, a light-emitting layer, anelectron-transport layer, and an electron-injection layer are stacked inthis order can be given as an embodiment. Note that not all of theselayers are necessarily provided in the EL layer 7014, and each of theselayers may be provided in duplicate or more. Furthermore, anothercomponent such as an electron-relay layer may be added as appropriate asan intermediate layer, in addition to a charge generation layer.

A light-emitting element 7012 is provided with a partition wall 7019which covers an edge of the first electrode 7017. As the partition wall7019, an inorganic insulating film or an organic polysiloxane film canbe applied in addition to an organic resin film of polyimide, acrylic,polyamide, epoxy, or the like. It is particularly preferable that thepartition wall 7019 be formed using a photosensitive resin material sothat a side surface of the partition wall 7019 is formed as a tiltedsurface with a continuous curvature. In the case where a photosensitiveresin material is used for the partition wall 7019, a step of forming aresist mask can be omitted. Further, the partition wall can be formedusing an inorganic insulating film. When the inorganic insulating filmis used for the partition wall, the amount of moisture included in thepartition wall can be reduced.

Note that a color filter layer 7033 is provided between thelight-emitting element 7012 and a substrate 7010 (see FIG. 19A). Astructure for emitting white light is employed for the light-emittingelement 7012, whereby light emitted from the light-emitting element 7012passes through the color filter layer 7033 and then passes through aninsulating film 7032, a gate insulating film 7031, an oxide insulatingfilm 7030, and the substrate 7010 so as to be emitted to the outside.

Plural kinds of the color filter layer 7033 may be formed. For example,a red color filter layer, a blue color filter layer, a green colorfilter layer can be provided in respective pixels. Note that the colorfilter layer 7033 is formed by a droplet discharge method such as aninkjet method, a printing method, an etching method using aphotolithography technique, or the like.

The color filter layer 7033 is covered with an overcoat layer 7034 and aprotective insulating film 7035 is further formed thereover. Note thatthe overcoat layer 7034 having a small thickness is illustrated in FIG.19A; the overcoat layer 7034 is formed using a resin material such as anacrylic resin and has a function of reducing unevenness due to the colorfilter layer 7033.

A contact hole which is formed in the insulating film 7032, the colorfilter layer 7033, the overcoat layer 7034, and the protectiveinsulating film 7035 and reaches the drain electrode is in a positionwhich overlaps with the partition wall 7019.

Next, a light-emitting element having a dual emission structure will bedescribed with reference to FIG. 19B. The light-emitting element havinga dual emission structure emits light in the directions indicated byarrows in FIG. 19B.

In FIG. 19B, an embodiment in which the n-channel transistor describedin Embodiment 1 is used as the light-emitting element driving transistor7021 is shown; however, one embodiment of the present invention is notparticularly limited thereto.

In FIG. 19B, an EL layer 7024 and a second electrode 7025 are stacked inthis order over a first electrode 7027 having a light-transmittingproperty, which is electrically connected to a source electrode or adrain electrode of the light-emitting element driving transistor 7021.

The first electrode 7027 and the second electrode 7025 are each formedusing a conductive film that transmits visible light. The material whichcan be used for the first electrode 7017 in FIG. 19A can be used for theconductive film that transmits visible light. Thus, the description ofthe first electrode 7017 is referred to for the details.

Note that one of the first electrode 7027 and the second electrode 7025functions as an anode, and the other functions as a cathode. It ispreferable to use a substance having a high work function for theelectrode which functions as an anode, and a substance having a low workfunction for the electrode which functions as a cathode.

The EL layer 7024 may be either a single layer or a stack of plurallayers. As for the EL layer 7024, the structure and material which canbe used for the EL layer 7014 in FIG. 19A can be used. Thus, thedescription of the EL layer 7014 is referred to for the details.

A light-emitting element 7022 is provided with a partition wall 7029which covers an edge of the first electrode 7027. As for the partitionwall 7029, the structure and material which can be used for thepartition wall 7019 in FIG. 19A can be used. Thus, the description ofthe partition wall 7019 is referred to for the details.

In addition, in the element structure illustrated in FIG. 19B, light isemitted from the light-emitting element 7022 to both the secondelectrode 7025 side and the first electrode 7027 side as indicated bythe arrows, and light emitted to the first electrode 7027 side passesthrough an insulating film 7042, a gate insulating film 7041, an oxideinsulating film 7040, and a substrate 7020 so as to be emitted to theoutside.

In the structure in FIG. 19B, for performing full-color display, thelight-emitting element 7022, one of light-emitting elements adjacent tothe light-emitting element 7022, and the other of the light-emittingelements are, for example, a green light-emitting element, a redlight-emitting element, and a blue light-emitting element, respectively.Alternatively, a light-emitting display device capable of full colordisplay may be manufactured using four kinds of light-emitting elementswhich include a white light-emitting element in addition to three kindsof light-emitting elements.

Next, a light-emitting element having a top emission structure will bedescribed with reference to FIG. 19C. The light-emitting element havinga top emission structure emits light in the direction indicated byarrows in FIG. 19C.

In FIG. 19C, an embodiment in which the n-channel transistor describedin Embodiment 1 is used as the light-emitting element driving transistor7001 is shown; however, one embodiment of the present invention is notparticularly limited thereto.

In FIG. 19C, an EL layer 7004 and a second electrode 7005 are stacked inthis order over a first electrode 7003 which is electrically connectedto a source electrode or a drain electrode of the light-emitting elementdriving transistor 7001.

As for the first electrode 7003, a material which efficiently reflectslight emitted from the EL layer 7004 is preferably used, in which casethe light extraction efficiency can be improved. Note that the firstelectrode 7003 may have a stacked-layer structure. For example, aconductive film that transmits visible light, which is formed on theside in contact with the EL layer 7004, may be stacked over alight-blocking film. As the light-blocking film, although a metal filmor the like which efficiently reflects light emitted from the EL layeris preferable, a resin or the like to which a black pigment is added canalso be used, for example.

The second electrode 7005 is formed using a conductive film thattransmits visible light. The material which can be used for the firstelectrode 7017 in FIG. 19A can be used for the conductive film thattransmits visible light. Thus, the description of the first electrode7017 is referred to for the details.

Note that one of the first electrode 7003 and the second electrode 7005functions as an anode, and the other functions as a cathode. It ispreferable to use a substance having a high work function for theelectrode which functions as an anode, and a substance having a low workfunction for the electrode which functions as a cathode.

The EL layer 7004 may be either a single layer or a stack of plurallayers. As for the EL layer 7004, the structure and material which canbe used for the EL layer 7014 in FIG. 19A can be used. Thus, thedescription of the EL layer 7014 is referred to for the details.

A light-emitting element 7002 is provided with a partition wall 7009which covers an edge of the first electrode 7003. As for the partitionwall 7009, the structure and material which can be used for thepartition wall 7019 in FIG. 19A can be used. Thus, the description ofthe partition wall 7019 is referred to for the details.

In FIG. 19C, the source electrode or the drain electrode of thelight-emitting element driving transistor 7001 is electrically connectedto the first electrode 7003 through a contact hole provided in a gateinsulating film 7051, a protective insulating film 7052, and aninsulating film 7055. A planarization insulating film 7053 can be formedusing a resin material such as polyimide, acrylic, benzocyclobutene,polyamide, or epoxy. Other than such resin materials, a low-dielectricconstant material (a low-k material), a siloxane-based resin, or thelike can be used. Note that the planarization insulating film 7053 maybe formed by stacking a plurality of insulating films formed using thesematerials. There is no particular limitation on the method for formingthe planarization insulating film 7053, and the planarization insulatingfilm 7053 can be formed, depending on the material, by a sputteringmethod, an SOG method, spin coating, dip coating, spray coating, adroplet discharge method (such as an inkjet method, screen printing, oroffset printing), or the like.

In the structure in FIG. 19C, for performing full-color display, thelight-emitting element 7002, one of light-emitting elements adjacent tothe light-emitting element 7002, and the other of the light-emittingelements are, for example, a green light-emitting element, a redlight-emitting element, and a blue light-emitting element, respectively.Alternatively, a light-emitting display device capable of full colordisplay may be manufactured using four kinds of light-emitting elementswhich include a white light-emitting element in addition to three kindsof light-emitting elements.

In the structure in FIG. 19C, a light-emitting display device capable offull color display may be manufactured in such a manner that all of aplurality of light-emitting elements which is arranged is whitelight-emitting elements and a sealing substrate having a color filter orthe like is arranged over the light-emitting element 7002. When amaterial which exhibits a single color such as white is formed andcombined with a color filter or a color conversion layer, full-colordisplay can be performed.

Needless to say, display of single color light emission may also beperformed. For example, a lighting device may be formed with the use ofwhite light emission, or an area-color light-emitting device may beformed with the use of single color light emission.

If necessary, an optical film such as a polarizing film including acircularly polarizing plate may be provided.

Note that an example is described in which a transistor that controlsthe driving of a light-emitting element (a light-emitting elementdriving transistor) is electrically connected to the light-emittingelement; however, a structure may be employed in which a currentcontrolling transistor is connected between the light-emitting elementdriving transistor and the light-emitting element.

The semiconductor device described in this embodiment is not limited tothe structures illustrated in FIGS. 19A to 19C and can be modified invarious ways based on the spirit of techniques of the present invention.

Embodiment 9

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including game machines). Examples ofelectronic devices are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone (also referred to as a cellularphone or a mobile phone device), a portable game machine, a portableinformation terminal, an audio reproducing device, and a large-sizedgame machine such as a pachinko machine. Embodiments of electronicdevices each including the display device described in the aboveembodiment will be described.

FIG. 20A illustrates a portable information terminal, which includes amain body 3001, a housing 3002, display portions 3003 a and 3003 b, andthe like. The display portion 3003 b is a panel having a touch-inputfunction. By touching keyboard buttons 3004 displayed on the displayportion 3003 b, a screen can be operated and text can be input. Needlessto say, the display portion 3003 a may be a panel having a touch-inputfunction. The liquid crystal panel or the organic light-emitting paneldescribed in Embodiment 8 is manufactured using the transistor describedin any of Embodiments 1 to 7 as a switching element and applied to thedisplay portion 3003 a or 3003 b, whereby the portable informationterminal can be obtained.

The portable information terminal illustrated in FIG. 20A can have afunction of displaying various kinds of information (e.g., a stillimage, a moving image, and a text image); a function of displaying acalendar, the date, the time, and the like on the display portion; afunction of operating or editing the information displayed on thedisplay portion; a function of controlling processing by various kindsof software (programs); and the like. Furthermore, an externalconnection terminal (such as an earphone terminal or a USB terminal), astorage medium insertion portion, and the like may be provided on theback surface or the side surface of the housing.

The portable information terminal illustrated in FIG. 20A may transmitand receive data wirelessly. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an electronicbook server.

Further, one of the two display portions 3003 a and 3003 b of theportable information terminal illustrated in FIG. 20A can be detached asshown in FIG. 20B. The display portion 3003 a can be a panel having atouch-input function, which contributes to further reduction in weightwhen it is carried around and to the convenience since operation can beperformed by one hand with the housing 3002 supported by the other hand.

Further, the housing 3002 illustrated in FIG. 20B may be equipped withan antenna, a microphone function, or a wireless communication functionto be used as a mobile phone.

FIG. 20C illustrates an embodiment of a mobile phone. A mobile phone5005 illustrated in FIG. 20C is provided with a display portion 5001incorporated in a housing, a display panel 5003 attached to a hinge5002, operation buttons 5004, a speaker, a microphone, and the like.

In the mobile phone 5005 illustrated in FIG. 20C, the display panel 5003is slid to overlap with the display portion 5001, and the display panel5003 also functions as a cover having a light-transmitting property. Thedisplay panel 5003 is a display panel including the light-emittingelement having a dual emission structure illustrated in FIG. 19B inEmbodiment 8, in which light emission is extracted through the surfaceopposite to the substrate side and the surface on the substrate side.

Since the light-emitting element having a dual emission structure isused for the display panel 5003, display can also be performed with thedisplay portion 5001 overlapped; therefore, both the display portion5001 and the display panel 5003 can perform display and a user can viewboth the displays. The display panel 5003 has a light-transmittingproperty and the view beyond the display panel can be seen. For example,when a map is displayed on the display portion 5001 and the locationpoint of the user is displayed using the display panel 5003, the presentlocation can be recognized easily.

Further, in the case where the mobile phone 5005 is provided with animage sensor to be used as a television telephone, it is possible tomake conversation with plural persons while their faces are displayed;therefore, a television conference or the like can be performed. Forexample, when the face of a single person or the faces of plural personsare displayed on the display panel 5003 and further the face of anotherperson is displayed on the display portion 5001, the user can makeconversation while viewing the faces of two or more persons.

When a touch input button 5006 displayed on the display panel 5003 istouched with a finger or the like, data can be input into the mobilephone 5005. In addition, operations such as making calls and composingmails can be conducted by sliding the display panel 5003 and touchingthe operation buttons 5004 with a finger or the like.

FIG. 20D illustrates an embodiment of a television set 9600. In thetelevision set 9600, a display portion 9603 is incorporated in a housing9601. The display portion 9603 can display images. Here, the housing9601 is supported on a stand 9605 provided with a CPU. When thetransistor described in any of Embodiments 1 to 7 is applied to thedisplay portion 9603, the television set 9600 can be obtained.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television set isconnected to a communication network with or without wires via themodem, one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver, between receivers, or the like) datacommunication can be performed.

Further, the television set 9600 is provided with an external connectionterminal 9604, a storage medium recording and reproducing portion 9602,and an external memory slot. The external connection terminal 9604 canbe connected to various types of cables such as a USB cable, and datacommunication with a personal computer or the like is possible. A diskstorage medium can be inserted into the storage medium recording andreproducing portion 9602, and reading data stored in the storage mediumand writing data into the storage medium can be performed. In addition,a picture, a video, or the like stored as data in an external memory9606 inserted into the external memory slot can be displayed on thedisplay portion 9603.

The methods, structures, and the like described in this embodiment canbe combined as appropriate with any of the methods, structures, and thelike described in the other embodiments.

EXPLANATION OF REFERENCE

101: substrate, 102: oxide insulating film, 103 a: oxide semiconductorfilm, 103 b: oxide semiconductor film, 103 c: oxide semiconductor film,104 a: oxide semiconductor film, 104 b: oxide semiconductor film, 104 c:oxide semiconductor film, 105: oxide semiconductor stack, 105 a: oxidesemiconductor film, 105 b: oxide semiconductor film, 105 c: oxidesemiconductor film, 106: electrode, 107: gate insulating film, 108: gateelectrode, 109: insulating film, 113 a: oxide semiconductor film, 113 b:oxide semiconductor film, 113 c: oxide semiconductor film, 114 a: oxidesemiconductor film, 114 b: oxide semiconductor film, 114 c: oxidesemiconductor film, 115: oxide semiconductor stack, 115 a: oxidesemiconductor film, 115 b: oxide semiconductor film, 115 c: oxidesemiconductor film, 116: electrode, 117: gate insulating film, 118: gateelectrode, 119: insulating film, 120: wiring, 123 b: oxide semiconductorfilm, 123 c: oxide semiconductor film, 124 b: oxide semiconductor film,124 c: oxide semiconductor film, 125: oxide semiconductor stack, 125 b:oxide semiconductor film, 125 c: oxide semiconductor film, 126:electrode, 127: gate insulating film, 128: gate electrode, 129:insulating film, 133 b: oxide semiconductor film, 133 c: oxidesemiconductor film, 134 b: oxide semiconductor film, 134 c: oxidesemiconductor film, 135: oxide semiconductor stack, 135 b: oxidesemiconductor film, 135 c: oxide semiconductor film, 136: electrode,137: gate insulating film, 138: gate electrode, 139: insulating film,147 a: gate insulating film, 147 b: gate insulating film, 148 a: gateelectrode, 148 b: gate electrode, 602: gate wiring, 603: gate wiring,616: source or drain electrode, 628: transistor, 629: transistor, 651:liquid crystal element, 652: liquid crystal element, 690: capacitorwiring, 2000: crystal structure, 2001: crystal structure, 3001: mainbody, 3002: housing, 3003 a: display portion, 3003 b: display portion,3004: keyboard button, 5001: display portion, 5002: hinge, 5003: displaypanel, 5004: operation button, 5005: mobile phone, 5006: touch inputbutton, 5300: substrate, 5301: pixel portion, 5302: scan line drivercircuit, 5303: scan line driver circuit, 5304: signal line drivercircuit, 6400: pixel, 6401: switching transistor, 6402: drivingtransistor, 6403: capacitor, 6404: light-emitting element, 6405: signalline, 6406: scan line, 6407: power supply line, 6408: common electrode,7001: light-emitting element driving transistor, 7002: light-emittingelement, 7003: electrode, 7004: EL layer, 7005: electrode, 7009:partition wall, 7010: substrate, 7011: light-emitting element drivingtransistor, 7012: light-emitting element, 7014: EL layer, 7015:electrode, 7016: film, 7017: electrode, 7019: partition wall, 7020:substrate, 7021: light-emitting element driving transistor, 7022:light-emitting element, 7024: EL layer, 7025: electrode, 7027:electrode, 7029: partition wall, 7030: oxide insulating film, 7031: gateinsulating film, 7032: insulating film, 7033: color filter layer, 7034:overcoat layer, 7035: protective insulating film, 7040: oxide insulatingfilm, 7041: gate insulating film, 7042: insulating film, 7051: gateinsulating film, 7052: protective insulating film, 7053: planarizationinsulating film, 7055: insulating film, 9600: television set, 9601:housing, 9602: storage medium recording and reproducing portion, 9603:display portion, 9604: external connection terminal, 9605: stand, and9606: external memory.

This application is based on Japanese Patent Application serial no.2010-267901 filed with the Japan Patent Office on Nov. 30, 2010 andJapanese Patent Application serial no. 2010-267896 filed with the JapanPatent Office on Nov. 30, 2010, the entire contents of which are herebyincorporated by reference.

1. A semiconductor device comprising: a first insulating film; a secondinsulating film overlapping with the first insulating film; a stack ofsemiconductor films interposed between the first insulating film and thesecond insulating film, the stack of semiconductor films comprising: afirst oxide semiconductor film having a first crystal structure; and asecond oxide semiconductor film having a second crystal structure, incontact with the first oxide semiconductor film and interposed betweenthe first oxide semiconductor film and the second insulating film; andan electrically conducting film overlapping with the stack ofsemiconductor films with the second insulating film interposedtherebetween, wherein the first crystal structure is a non-wurtzitestructure or a deformed structure of a non-wurtzite structure; andwherein the second crystal structure is a wurtzite structure.
 2. Asemiconductor device according to claim 1, wherein the first crystalstructure is one of a YbFe₂O₄ structure, a Yb₂Fe₃O₇ structure, adeformed structure of a YbFe₂O₄ structure, and a deformed structure of aYb₂Fe₃O₇.
 3. A semiconductor device according to claim 1, wherein aconcentration in nitrogen of the second oxide semiconductor film ishigher than a concentration in nitrogen of the first oxide semiconductorfilm.
 4. A semiconductor device according to claim 1, the stack ofsemiconductor films further comprising a third oxide semiconductor filmhaving a third crystal structure interposed between the second oxidesemiconductor film and the second insulating film, wherein the thirdcrystal structure is a non-wurtzite structure or a deformed structure ofa non-wurtzite structure.
 5. A semiconductor device according to claim4, wherein the first crystal structure is one of a YbFe₂O₄ structure, aYb₂Fe₃O₇ structure, a deformed structure of a YbFe₂O₄ structure, and adeformed structure of a Yb₂Fe₃O₇.
 6. A semiconductor device according toclaim 1, wherein the first oxide semiconductor film has a trigonal orhexagonal structure film.
 7. A semiconductor device according to claim1, wherein the first oxide semiconductor film and the second oxidesemiconductor film are non-single-crystals and comprise an amorphousregion and a crystal region having c-axis alignment.
 8. A semiconductordevice according to claim 1, wherein the first oxide semiconductor filmcomprises zinc, indium, or gallium.
 9. A semiconductor device accordingto claim 1, wherein the second oxide semiconductor film is a zinc oxideor an oxynitride semiconductor.
 10. An electronic device including thesemiconductor device according to claim
 1. 11. A method formanufacturing a semiconductor device, the method comprising the stepsof: providing a substrate having an electrically insulating top surface;forming a first oxide semiconductor film over the substrate in a firstatmosphere; forming a second oxide semiconductor film on and in contactwith the first oxide semiconductor film in a second atmosphere having ahigher concentration in nitrogen than the first atmosphere; performingheat treatment to the first oxide semiconductor film and the secondoxide semiconductor film so that the first oxide semiconductor film iscrystallized in a first crystal structure and the second oxidesemiconductor film is crystallized in a second crystal structure,wherein the first crystal structure is a non-wurtzite structure or adeformed structure of a non-wurtzite structure; and wherein the secondcrystal structure is a wurtzite structure.
 12. A method formanufacturing a semiconductor device according to claim 11, wherein thefirst crystal structure is one of a YbFe₂O₄ structure, a Yb₂Fe₃O₇structure, a deformed structure of a YbFe₂O₄ structure, and a deformedstructure of a Yb₂Fe₃O₇.
 13. A method for manufacturing a semiconductordevice according to claim 11, wherein a concentration in nitrogen of thesecond oxide semiconductor film is higher than a concentration innitrogen of the first oxide semiconductor film.
 14. A method formanufacturing a semiconductor device according to claim 11, furthercomprising the steps of: forming a third oxide semiconductor film on andin contact with the second oxide semiconductor film; and performing anadditional heat treatment on the third oxide semiconductor film so thatthe third oxide semiconductor film is crystallized in a third crystalstructure, wherein the third crystal structure is a non-wurtzitestructure or a deformed structure of a non-wurtzite structure.
 15. Amethod for manufacturing a semiconductor device according to claim 14,wherein the third crystal structure is one of a YbFe₂O₄ structure, aYb₂Fe₃O₇ structure, a deformed structure of a YbFe₂O₄ structure, and adeformed structure of a Yb₂Fe₃O₇.
 16. A method for manufacturing asemiconductor device according to claim 11, wherein the first oxidesemiconductor film has a trigonal or hexagonal structure film.
 17. Amethod for manufacturing a semiconductor device according to claim 11,wherein the first oxide semiconductor film and the second oxidesemiconductor film are non-single-crystals and comprise an amorphousregion and a crystal region having c-axis alignment.
 18. A method formanufacturing a semiconductor device according to claim 11, wherein thefirst oxide semiconductor film comprises zinc, indium, or gallium.
 19. Amethod for manufacturing a semiconductor device according to claim 11,wherein the second oxide semiconductor film is a zinc oxide or anoxynitride semiconductor.
 20. A method for manufacturing a semiconductordevice according to claim 11, wherein the first oxide semiconductor filmand the second oxide semiconductor film are formed successively by asputtering method; and wherein nitrogen is introduced in a formationchamber to form the second oxide semiconductor film after that the firstoxide semiconductor film has been formed.